Expose addSourcePaths function for directory dependencies

[DSteve595]

This adds fun addSourcePaths(sourcePaths: Any) to the plugin's extension. It can be used when including source directories for generated code, which often come as FileCollections with task dependencies. Using a ConfigurableFileCollection as a task input lets us automatically run those dependency tasks when applicable.

A practical example is including a KotlinSourceSet's source directories in the signature file:

metalava {
  val consumerSourceSetNames = setOf("androidMain", "desktopMain")
  val kotlinSourceFileCollection = files(paths = emptyArray<Any>()) {
    consumerSourceSetNames
      .flatMap {
        val sourceSet = kotlin.sourceSets.getByName(it)
        sourceSet.dependsOn + sourceSet
      }
      .forEach { from(it.kotlin.sourceDirectories) }
  }

  addSourcePaths(kotlinSourceFileCollection)
}

This will ensure even generated code properly gets seen by Metalava.

[DSteve595]

Superseded by #31, which handles this in a less manual way. Technically less flexible, but I can't think of any use cases for adding a source path that isn't on the compile classpath.

[DSteve595]

Actually, reopening since this would currently be the only way to add a directory that isn't named "java"/"kotlin". That restriction is a bit unexpected.

[tylerbwong]

Looks good, just needs a formatting fix and ./gradlew metalavaGenerateSignature to reflect the new API.