Track random access to source files in Program

[ajafff]

Follow up to #370

Rules that access files on Program, which are not global or a (transitive) dependency of the current file, currently don't work well with caching. That's because the cache is not invalidated if such an implicit dependency changes.

We can do that by building a small wrapper for Program that tracks access to getSourceFile(byPath), getSourceFiles, get*Diagnostics without sourceFile parameter.

TBD:

• getTypeCatalog
• getCurrentDirectory
• getRootFileNames