Less confusing error message (backport #4789)

integration/failure/module-outside-top-level-module/src/ModuleOutsideTopLevelModuleTests.scala

@@ -12,7 +12,7 @@ object ModuleOutsideTopLevelModuleTests extends UtestIntegrationTestSuite {
       assert(!res.isSuccess)
       assert(
         res.err.contains(
-          "Modules and Tasks can only be defined within a mill Module"
+          "Modules and Tasks can only be defined within a mill Module (in `build.mill` or `package.mill` files)"
         )
       )
       assert(

integration/failure/no-modules-in-helper-file/src/NoModulesInHelperFileTests.scala

@@ -11,7 +11,9 @@ object NoModulesInHelperFileTests extends UtestIntegrationTestSuite {
       val res = eval(("resolve", "_"))
       assert(res.isSuccess == false)
       assert(
-        res.err.contains("Modules and Tasks can only be defined within a mill Module")
+        res.err.contains(
+          "Modules and Tasks can only be defined within a mill Module (in `build.mill` or `package.mill` files)"
+        )
       )
       assert(res.err.contains("object foo extends Module"))
     }

main/define/src/mill/define/Ctx.scala

@@ -18,7 +18,9 @@ import scala.annotation.{compileTimeOnly, implicitNotFound}
  * @param enclosingCls
  * @param crossValues
  */
-@implicitNotFound("Modules and Tasks can only be defined within a mill Module")
+@implicitNotFound(
+  "Modules and Tasks can only be defined within a mill Module (in `build.mill` or `package.mill` files)"
+)
 trait Ctx {
   def enclosing: String
   def lineNum: Int
@@ -112,7 +114,7 @@ trait LowPriCtx {
   // as it is provided by the codegen. Defined for IDEs to think that one is available
   // and not show errors in build.mill/package.mill even though they can't see the codegen
   @compileTimeOnly(
-    "Modules and Tasks can only be defined within a mill Module"
+    "Modules and Tasks can only be defined within a mill Module (in `build.mill` or `package.mill` files)"
   )
   implicit def dummyInfo: Ctx = sys.error("implicit Ctx must be provided")
 }