Allow preprocessor-specific reorderings

[vmchale]

This is a slimmed-down version of #6233 that allows users to define a reordering function when they define a pre-processor, based on @DanielG's suggestion.

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• If the change is docs-only, [ci skip] is used to avoid triggering the build bots.

This was not tested; I do know it "intervenes" in the right place by the past pull request. It is one step towards handling c2hs correctly.

[DanielG]

Test failure:

/home/travis/build/haskell/cabal/cabal-testsuite/PackageTests/CustomPreProcess/Setup.hs:18:7: warning: [-Wmissing-fields]
    * Fields of `PreProcessor' not initialised: reorderPreProcessorDeps
    * In the expression:
        PreProcessor
          {platformIndependent = True,
           runPreProcessor = mkSimplePreProcessor
                               $ \ inFile outFile verbosity
                                   -> do info
                                           verbosity
                                           ("Preprocessing " ++ inFile ++ " to " ++ outFile)
                                         ....}
      In an equation for `myCustomPreprocessor':
          myCustomPreprocessor _bi lbi _clbi
            = PreProcessor
                {platformIndependent = True,
                 runPreProcessor = mkSimplePreProcessor
                                     $ \ inFile outFile verbosity -> do ...}
            where
                builddir = buildDir lbi
                progName = "my-custom-preprocessor"
                progPath = builddir </> progName </> progName
      In an equation for `main':
          main
            = defaultMainWithHooks
                simpleUserHooks
                  {hookedPreProcessors = [("pre", myCustomPreprocessor)]}
            where
                myCustomPreprocessor ::
                  BuildInfo
                  -> LocalBuildInfo -> ComponentLocalBuildInfo -> PreProcessor
                myCustomPreprocessor _bi lbi _clbi
                  = PreProcessor
                      {platformIndependent = True,
                       runPreProcessor = mkSimplePreProcessor
                                           $ \ inFile outFile verbosity -> ...}
                  where
                      builddir = buildDir lbi
                      progName = "my-custom-preprocessor"
                      ....
                callProcess :: FilePath -> [String] -> IO ()
                callProcess path args
                  = do exitCode <- rawSystem path args
                       ....
   |
18 |       PreProcessor {
   |       ^^^^^^^^^^^^^^...

[vmchale]

Btw if others have further suggestions about design, I am open to other things here.

[vmchale]

@DanielG thoughts? It is now passing CI.