Remove remaining Gradle internal APIs, if possible

[DPUkyle]

As a follow-up to #39, there are a handful of additional places where I have no choice (AFAICT) but to use Gradle internal APIs.

Here are the remaining usages as of v0.3.7 and the justification for them.

org.gosulang.gradle.GosuBasePlugin

Internal APIs used:

• org.gradle.api.internal.file.SourceDirectorySetFactory
• org.gradle.api.internal.tasks.DefaultSourceSetOutput
• org.gradle.internal.Cast

Justification:

• SourceDirectorySetFactory use is relatively safe here, as it is injected by DI and passed to the DefaultGosuSourceSet constructor. More on that later.
• Cast is used to coerce a SourceSetOutput instance to DefaultSourceSetOutput so that I may access the addClassesDir(Callable<File>) method. Normally this is done safely inside the org.gradle.api.plugins.internal.SourceSetUtil#configureOutputDirectoryForSourceSet method, but since SourceSetUtil is also internal I decided to make copies of the methods. But perhaps a better solution is to expose SourceSetUtil as public?

org.gosulang.gradle.tasks.DefaultGosuSourceSet

Internal APIs used:

• org.gradle.api.internal.file.SourceDirectorySetFactory

Justification:

• As mentioned above, I call the create(String, String) method on an injected reference to SourceDirectorySetFactory. I then set reasonable defaults on the created SourceDirectorySets.

org.gosulang.gradle.tasks.GosuRuntime

Internal APIs used:

• org.gradle.api.internal.file.collections.LazilyInitializedFileCollection
• org.gradle.api.internal.tasks.TaskDependencyResolveContext

Justification:

• This code is very similar to org.gradle.api.tasks.ScalaRuntime, which dynamically adds a dependency on artifact org.scala-lang:scala-compiler based on the version of existing dependency org.scala-lang:scala-library. Just like that code, GosuRuntime needs to perform late evaluation of the dependencies or it will fail if run too soon in the build.

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If there are better ways to do any or all of the above without depending on internal APIs, I would welcome help/advice on how to isolate this plugin from core changes. It's a win-win!

[DPUkyle]

Some references to org.gradle.util still exist and should be removed, but overall the risk footprint of internal API usage is nearly eliminated.