Support for `module-info.java` in separate folder

[weisJ]

Currently the module-info.java file needs to be in the same directory as the other sources. For use with mixedJavaRelease this is rather unfortunate as I now have to either use my IDE with the language level set to 9 (even though I am only targeting 8) or 8, which causes the IDE to report an error for the module-info.java file.

It would be great to have the option to place the module-info.java file in a separate folder, which ideally would be marked as a source set with the appropriate sourceCompatibility passed to mixedJavaRelease.

[big-andy-coates]

This project is now maintained by the community. Please feel free to raise a PR is this is a feature you'd like to see added.