Separate compilation of module-info.java

[tlinkowski]

A proof of concept for #72 (actually, it's a fully working solution, but since #72 hasn't been discussed yet, I call it a proof of concept for now).

This PR consists of four clearly separated commits (every commit passes the build):

1. initial refactorings/improvements
   • no new features introduced - necessary to make further commits much clearer
   • bugfix: renamed TestModuleOptions.isRunOnClasspath() to getRunOnClasspath() (otherwise it doesn't work with Kotlin DSL)
   • commit details
2. support for moduleOptions.compileModuleInfoSeparately (low-level API)
   • when moduleOptions.compileModuleInfoSeparately = true, introduces a special compileModuleInfoJava task → boils down to (Groovy DSL):

     compileJava {
       exclude 'module-info.java'
     }
     
     tasks.register('compileModuleInfoJava', JavaCompile) {
       classpath = files()
       source = 'src/main/java/module-info.java'
       destinationDir = compileJava.destinationDir
     }
     
     compileModuleInfoJava.dependsOn compileJava
     classes.dependsOn compileModuleInfoJava

     + modular behavior is configured on compileModuleInfoJava instead of compileJava (but always using compileJava's class-path as module-path)
   • module-info.class location: root output directory
   • NOTE: introduces CompileModuleOptions extends ModuleOptions, which is a potentially breaking change for compileJava in Kotlin DSL (see modified build.gradle.kts)
   • commit details
3. support for modularity project extension (high-level API)
   • exposed via ModularityExtension interface
   • sets appropriate --release options for compileJava and compileModuleInfoJava
   • commit details
4. added test-project-mixed
   • tests modularity.mixedJavaRelease and modularity.standardJavaRelease
   • the test verifies generated class file versions
   • project is "mixed" in two senses:
     1. classes target various JDKs
     2. build files are in Groovy DSL & Kotlin DSL
   • I managed to get rid of local_maven_build.gradle and smoke_test_build.gradle by using pluginManagement in *_settings.gradle files (can be ported to other test projects)
   • commit details

PS. If you decide to merge this PR, I'd appreciate if you didn't squash the commits (they are separated on purpose, to provide a clear change history).

[paulbakker]

Finally had time to review this. Really excellent work!
I have a few very minor suggestions and questions, but other than that I'm happy to merge.

[paulbakker]

Add some more description why this is useful and when to use which option. You explained this really well in the issue, maybe move some of that to the docs.

[tlinkowski]

OK, will do.

[tlinkowski]

I added more description, although I didn't copy it from the issue, because it seemed to me I already copied all the necessary stuff to the "Separate compilation of module-info.java" section.

Please, let me know if the updated version reads well. I'll gladly reword/restructure it further if you provide me with any remarks you might have.

[tlinkowski]

Thanks for your feedback, Paul! I'll provide the requested changes within a few days.

Please, let me know if you prefer:

• a force push with modified commits (total of 4, as now),
• or just one more commit (total of 5).

As I mentioned, I'd much prefer that the commits aren't squashed for clear change history.

[paulbakker]

I'm fine with either, whatever you think shows intentions best.

[tlinkowski]

I force-pushed the new version. Please, review.

PS. I also rebased it to the current master so that you don't have to resolve a syntactic conflict (visibility of ModuleOptions.mutateArgs) that occurred due to @aalmiray's last merged PR (#74).