[Do not merge] Testing building with Java 9 in our CI environment.

[retronym]

I've run sbt publishLocal on the CI server to make SBT 0.13.4-b646662
available. This includes changes to be compatible with Java 9.

I've also installed JDK9-ea on the CI server, and made it selectable
with jvmSelect oracle 9:

scala/scala-jenkins-infra#221

Finally, I've used:

https://github.com/retronym/java9-rt-export

to export a Java 8-style rt.jar to make scalac work without
needing native support for reading the jrt:// virtual filesystem
containing the standard library classes. Scala 2.12.1 and higher
support this, but SBT itself still uses Scala 2.10.6, so the shim
JAR is required.

This commit uses 0.13.4-b646662 for this build, and sets an system
property to instruct scalac to add the rt.jar shim to the compilation
classpath.

[retronym]

The CI builds in this PR will still be using OpenJDK 8. That failed with:

++ /usr/local/bin/sbt -sbt-version 0.13.14-b646662 -Dscala.ext.dirs=/usr/lib/jvm/jdk-9-ea-oracle/scala-ext -Dsbt.override.build.repos=true -Dsbt.repository.config=/home/jenkins/workspace/scala-2.12.x-validate-publish-core/scripts/sbt-repositories-config --warn 'setupPublishCore https://scala-ci.typesafe.com/artifactory/scala-pr-validation-snapshots/' generateBuildCharacterPropertiesFile
Java HotSpot(TM) 64-Bit Server VM warning: Ignoring option MaxPermSize; support was removed in 8.0
java.lang.AssertionError: assertion failed: not a type: symbol
 loaded from class BuildSettings$ in package with name BuildSettings$ and classloader java.net.URLClassLoader@1148eca4
	at scala.reflect.runtime.JavaMirrors$JavaMirror.scala$reflect$runtime$JavaMirrors$JavaMirror$$classToScala1(JavaMirrors.scala:979)
	at scala.reflect.runtime.JavaMirrors$JavaMirror$$anonfun$classToScala$1.apply(JavaMirrors.scala:938)
	at scala.reflect.runtime.JavaMirrors$JavaMirror$$anonfun$classToScala$1.apply(JavaMirrors.scala:938)
	at scala.reflect.runtime.JavaMirrors$JavaMirror$$anonfun$toScala$1.apply(JavaMirrors.scala:104)
	at scala.reflect.runtime.TwoWayCache.toScala(TwoWayCache.scala:38)
	at scala.reflect.runtime.JavaMirrors$JavaMirror.toScala(JavaMirrors.scala:102)
	at scala.reflect.runtime.JavaMirrors$JavaMirror.classToScala(JavaMirrors.scala:938)
	at scala.reflect.runtime.JavaMirrors$JavaMirror.classSymbol(JavaMirrors.scala:206)
	at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaInstanceMirror.symbol(JavaMirrors.scala:245)
	at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaInstanceMirror.symbol(JavaMirrors.scala:243)
	at sbt.Plugins$$anonfun$1.apply$mcZ$sp(Plugins.scala:349)
	at sbt.Plugins$$anonfun$1.apply(Plugins.scala:349)
	at sbt.Plugins$$anonfun$1.apply(Plugins.scala:349)
	at scala.util.control.Exception$Catch$$anonfun$opt$1.apply(Exception.scala:118)
	at scala.util.control.Exception$Catch$$anonfun$opt$1.apply(Exception.scala:118)
	at scala.util.control.Exception$Catch.apply(Exception.scala:102)
	at scala.util.control.Exception$Catch.opt(Exception.scala:118)
	at sbt.Plugins$.hasAutoImportGetter(Plugins.scala:348)
	at sbt.PluginDiscovery$$anonfun$1.apply(PluginDiscovery.scala:41)
	at sbt.PluginDiscovery$$anonfun$1.apply(PluginDiscovery.scala:39)
	at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
	at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
	at scala.collection.immutable.List.foreach(List.scala:318)
	at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
	at scala.collection.AbstractTraversable.map(Traversable.scala:105)
	at sbt.PluginDiscovery$.discoverAll(PluginDiscovery.scala:39)
	at sbt.Load$.loadPlugins(Load.scala:900)
	at sbt.Load$.loadPluginDefinition(Load.scala:857)
	at sbt.Load$.buildPlugins(Load.scala:852)
	at sbt.Load$.plugins(Load.scala:840)
	at sbt.Load$$anonfun$loadUnit$1$$anonfun$34.apply(Load.scala:465)
	at sbt.Load$$anonfun$loadUnit$1$$anonfun$34.apply(Load.scala:465)
	at sbt.Load$.timed(Load.scala:1025)
	at sbt.Load$$anonfun$loadUnit$1.apply(Load.scala:464)
	at sbt.Load$$anonfun$loadUnit$1.apply(Load.scala:459)
	at sbt.Load$.timed(Load.scala:1025)
	at sbt.Load$.loadUnit(Load.scala:459)
	at sbt.Load$$anonfun$25$$anonfun$apply$14.apply(Load.scala:311)
	at sbt.Load$$anonfun$25$$anonfun$apply$14.apply(Load.scala:310)
	at sbt.BuildLoader$$anonfun$componentLoader$1$$anonfun$apply$4$$anonfun$apply$5$$anonfun$apply$6.apply(BuildLoader.scala:91)
	at sbt.BuildLoader$$anonfun$componentLoader$1$$anonfun$apply$4$$anonfun$apply$5$$anonfun$apply$6.apply(BuildLoader.scala:90)
	at sbt.BuildLoader.apply(BuildLoader.scala:140)
	at sbt.Load$.loadAll(Load.scala:365)
	at sbt.Load$.loadURI(Load.scala:320)
	at sbt.Load$.load(Load.scala:316)
	at sbt.Load$.load(Load.scala:305)
	at sbt.Load$$anonfun$4.apply(Load.scala:146)
	at sbt.Load$$anonfun$4.apply(Load.scala:146)
	at sbt.Load$.timed(Load.scala:1025)
	at sbt.Load$.apply(Load.scala:146)
	at sbt.Load$.defaultLoad(Load.scala:39)
	at sbt.BuiltinCommands$.liftedTree1$1(Main.scala:496)
	at sbt.BuiltinCommands$.doLoadProject(Main.scala:496)
	at sbt.BuiltinCommands$$anonfun$loadProjectImpl$2.apply(Main.scala:488)
	at sbt.BuiltinCommands$$anonfun$loadProjectImpl$2.apply(Main.scala:488)
	at sbt.Command$$anonfun$applyEffect$1$$anonfun$apply$2.apply(Command.scala:59)
	at sbt.Command$$anonfun$applyEffect$1$$anonfun$apply$2.apply(Command.scala:59)
	at sbt.Command$$anonfun$applyEffect$2$$anonfun$apply$3.apply(Command.scala:61)
	at sbt.Command$$anonfun$applyEffect$2$$anonfun$apply$3.apply(Command.scala:61)
	at sbt.Command$.process(Command.scala:93)
	at sbt.MainLoop$$anonfun$1$$anonfun$apply$1.apply(MainLoop.scala:96)
	at sbt.MainLoop$$anonfun$1$$anonfun$apply$1.apply(MainLoop.scala:96)
	at sbt.State$$anon$1.process(State.scala:184)
	at sbt.MainLoop$$anonfun$1.apply(MainLoop.scala:96)
	at sbt.MainLoop$$anonfun$1.apply(MainLoop.scala:96)
	at sbt.ErrorHandling$.wideConvert(ErrorHandling.scala:17)
	at sbt.MainLoop$.next(MainLoop.scala:96)
	at sbt.MainLoop$.run(MainLoop.scala:89)
	at sbt.MainLoop$$anonfun$runWithNewLog$1.apply(MainLoop.scala:68)
	at sbt.MainLoop$$anonfun$runWithNewLog$1.apply(MainLoop.scala:63)
	at sbt.Using.apply(Using.scala:24)
	at sbt.MainLoop$.runWithNewLog(MainLoop.scala:63)
	at sbt.MainLoop$.runAndClearLast(MainLoop.scala:46)
	at sbt.MainLoop$.runLoggedLoop(MainLoop.scala:30)
	at sbt.MainLoop$.runLogged(MainLoop.scala:22)
	at sbt.StandardMain$.runManaged(Main.scala:57)
	at sbt.xMain.run(Main.scala:29)
	at xsbt.boot.Launch$$anonfun$run$1.apply(Launch.scala:109)
	at xsbt.boot.Launch$.withContextLoader(Launch.scala:128)
	at xsbt.boot.Launch$.run(Launch.scala:109)
	at xsbt.boot.Launch$$anonfun$apply$1.apply(Launch.scala:35)
	at xsbt.boot.Launch$.launch(Launch.scala:117)
	at xsbt.boot.Launch$.apply(Launch.scala:18)
	at xsbt.boot.Boot$.runImpl(Boot.scala:41)
	at xsbt.boot.Boot$.main(Boot.scala:17)
	at xsbt.boot.Boot.main(Boot.scala)
[error] java.lang.AssertionError: assertion failed: not a type: symbol
[error]  loaded from class BuildSettings$ in package with name BuildSettings$ and classloader java.net.URLClassLoader@1148eca4

This might be a regression in SBT since 0.13.13, or some problem in my locally built version of SBT. /cc @dwijnand @eed3si9n

Once we solve this, I'll manually build with Java 9 by running the Jenkins job with a custom JDK selection parameter.

[retronym]

I've minimized the exception in runtime reflection and lodged it as scala/scala-dev#304. The workaround is to avoid the empty package in the build definition.