Static analysis tools need additional information to help understand Python targets correctly. The use case of particular note are type checkers.

See discussion #2455_

At a minimum, these tools need the original source files. For type checkers, pyi files are also needed. Similarly, things like proto (or other code gen) would also benefit from a way to provide pyi files.

• Add PyInfo fields: {direct, transitive} x {original_sources, pyi_files}
• Add py_library.{pyi_{srcs, deps}
• Update py_library et al to populate, merge, propagate this information
• Update pypi generation code to set attributes appropriately.