skip non-class files in the exclusions processing for MergeJars

[vinnybod]

Note: This is a dupe of #1125 . I had to reopen the PR from a different GH org.

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This is sort of an addendum to #1106 and #1107
In the previous PR, I added an exception for COPYRIGHT and NOTICE since it was the same kind of use case as the existing LICENSE exception.

The issue that I am having is that I have files that should be present in all of the jars being published in a repo that are not COPYRIGHT, NOTICE, or LICENSE. For example a project-version.properties file that contains info about the build.

Changing the readExcludedFileNames function to only process *.class files allows for these files to propagate to the final jar. It also still passes all existing test cases.

The only test for this functionality is specifically looking at .class files.https://github.com/bazelbuild/rules_jvm_external/blob/master/tests/com/github/bazelbuild/rules_jvm_external/jar/MergeJarsTest.java#L221

I am open to other solutions, but this seems to have fixed a lot of issues for my use case without breaking any existing functionality that is being tested for.

[shs96c]

Let's see what happens. LGTM.