Handle no known candidate warnings when signature file is found

[mpilgrem]

The build of integration test backpack-x-pkg-transitive sees warnings:

Warning: Unable to find a known candidate for the Cabal entry Logger, but did find:
         * Logger.hsig
         * Logger.hsig 
          
         If you are using a custom preprocessor for this module with its own file extension,
         consider adding the extension to the value of the custom-preprocessor-extensions key in Stack's
         project-level configuration file (stack.yaml).

Warning: Unable to find a known candidate for the Cabal entry Str, but did find:
         * Str.hsig 
          
         If you are using a custom preprocessor for this module with its own file extension,
         consider adding the extension to the value of the custom-preprocessor-extensions key in Stack's
         project-level configuration file (stack.yaml).

generated by (pre-Backpack support) Stack.ComponentFile.logPossibilities - see findCandidate, resolveFiles, resolveFilesAndDeps, resolveComponentFiles, stackLibraryFiles (and similar), Stack.PackageFile.packageDescModulesAndFiles and getPackageFile.

In the context of Backpack support, these warnings could be more helpful.

---

We have Stack.Constants.haskellDefaultPreprocessorExts:

-- | Extensions for modules that are preprocessed by common preprocessors.
haskellDefaultPreprocessorExts :: [Text]
haskellDefaultPreprocessorExts = ["gc", "chs", "hsc", "x", "y", "ly", "cpphs"]

and findCandidate (extract):

  customPreprocessorExts <- view $ configL . to (.customPreprocessorExts)
  let haskellPreprocessorExts =
        haskellDefaultPreprocessorExts ++ customPreprocessorExts
  liftIO (makeNameCandidates haskellPreprocessorExts) >>= \case ...

Should .hsig also be treated as if it were an extension of something that will be 'preprocessed'?

Pinging @philippedev101 for his views.