Move private definitions out of //bazel

This is needed to make protobuf/bazel package minimal for other proto rules.

Keep 4 public bzl files in upb/bazel. They end up under protobuf/bazel, and they are legitimately used by other repositories.

Move upb_proto_library_internal/* under bazel/private. Those are utilities used in the rules. Moving them one level deeper makes protobuf/bazel package clean for other rules.

Move build_defs.bzl and amalgamation under /upb/bazel. Those are utilities used in the build.

Move lua.BUILD and python* uner /python/dist. Those are used in the WORKSPACE dependency setup.

PiperOrigin-RevId: 621442236