Warn if generating a `.java` or `.kt` file when an equivalent `.class` file already exists

We’ve seen problems where projects use sourcePath when they want protoPath, and end up getting multiple generated files for the same .proto input. Often this is benign, but it will cause problems if one generator is .java and the other is .kt.

When generating say, com/example/Foo.java, we should check to see if a file com/example/Foo.class exists on either the sourcePath or the protoPath. If it does, we should emit a warning:

  Warning: generated files duplicate existing classes.
     com.example.Foo duplicates a class in com/example/foo-1.0.0.jar
     com.bananas.Banana duplicates a class in com/bananas/banana-2.0.0.jar
  Use protoPath {...} rather than sourcePath {...} to not generate classes for these protos.
  Or use `wire_package` to avoid collisions with protoc-generated classes.
  Or use `checkForDuplicates=false` to suppress this warning.

The issue does not name specific files or tests; start by locating the code-generation path that handles Java and Kotlin output and the sourcePath/protoPath options. Use the duplicate-file examples and warning text as acceptance criteria, including the checkForDuplicates suppression option and the suggested remedies.