`JavaRenderer` should receive `SourceFileSet` only with `.java` files

[alexander-yevsyukov]

This issue is related to #17, but occurs to JavaRenderers. For example, in mc-java I had to add the below code so that only Java files are processed:

override fun render(sources: SourceFileSet) {
    // We could receive `grpc` or `kotlin` output roots here. Now we do only `java`.
    if (!sources.outputRoot.endsWith("java")) {
        return
    }
    ...
}       

The manipulation with the name of the output root works here because I know for sure that grpc directory is not going to have rejection types. But in general we need to take care about the content of the file set. See io.spine.tools.mc.java.rejection.RejectionRenderer for details.

As a developer I expect that only .java files are passed to a descendant of the class called JavaRenderer.

Prior work

There was an attempt to address this issue in ProtoData.
We need to analyse this PR in terms of the suggested terminology and implementation approach.

[alexander-yevsyukov]

@dmdashenkov, for your attention in the light of our discussion on responsibilities of the SourceFileSet class.

[alexander-yevsyukov]

We already have filtering which allows writing the code like this:

    override fun render(sources: SourceFileSet) {
        sources.forEachOfLanguage<Java> {
            // Do the job about `it`.
        }
    }

This way we 1) have convenient enough filtering and 2) do not limit the given SourceFileSet to contain files of only one language.