Jarjar shouldn't mangle classes when no meaningful transformation applied

[cdkrot]

In case big module and all it dependent jars are going through jarjar shading, there will probably be dependent jars which are not actually modified by shading.

However because they go through jarjar, it unpacks, revisits, and repacks the class back resulting a technically different class from before shading (even class size can be different from original). This upsets classpath checkers (i.e. checking that runtime classpath has at most 1 unique class version).

I propose to add a change so that jarjar can detect whether the meaningful transformation to class file has happened and if not, the class is unmodified.

This pull request introduces following new concepts:

• TracingRemapper: extension of Remapper with capability of tracking if the change was applied;
• RemappingJarProcessor: extension of JarProcessor focusing on TracingRemapper-based processors. It assumes ownership of TracingRemapper, and will verify if the mentioned remapper did any transformation.

The existing classes change as follows:

• PackageRemapper: becomes instance of TracingRemapper.
• JarTransformer, MethodSignatureProcessor: become instance of RemappingJarProcessor.
• JarTransformerChain: instead of being stateful class keeping track of parent visitors, becomes stateless class. This potentially allows multi-threaded processing and better design overall.

Added coverage for the proposed new functionality in UnmodifiedTest.

Closes: #47

[eed3si9n]

Could you run scalafmt plz?

[warn] /home/runner/work/jarjar-abrams/jarjar-abrams/core/src/main/scala/com/eed3si9n/jarjar/JJProcessor.scala isn't formatted properly!
[error] 1 files must be formatted
[error] (core / Compile / scalafmtCheck) 1 files must be formatted

[cdkrot]

Thanks! I think I did now