Fine-grained selective testing at a class-level granularity

[lihaoyi]

#4091 implements selective execution and testing at a granularity of mill build modules, but we should be able to do better.

• We already have the ability to compute code signatures for JVM bytecode and do basic callgraph analysis, which we currently use for selectively invalidating targets in the build.mill file.

• We should be able to apply the same bytecode analysis to classes and methods in the JavaModule and ScalaModule compile output, automatically determining which test classes within a module could potentially be affected by a code change and would need to be executed to validate it

This would provide much finer grained selective testing than the initial module-level implementation, and help greatly in messy real-world projects where the module structure is not as fine grained as it could be