Fix LazyClassKey proguard file issues.

[copybara-service]

Fix LazyClassKey proguard file issues.

• Fixes the name of the proguard file to contain an underscore between the package and simple names (e.g. test.FooModule was testFooModule_LazyClassKeys.pro and is now test_FooModule_LazyClassKeys.pro).
• Fixes incremental processing by adding the originating element to the writeResource call ([KSP] Incremental symbol processing not working (or working poorly) #4549).
• Fixes bug where StringBuilder was declared outside module for-loop, which could lead to duplicate entries across proguard rules for different modules.
• Fixed ClassName#toString() usage to ClassName#canonicalName(), since toString() is ambiguous and will silently break things when we migrate to XClassName.
• Added test coverage for the proguard file name and contents.

Fixes #4549

RELNOTES=Fixes #4549: Fixes incremental processing for LazyClassKey proguard files by adding the originating element to the writeResource call.