This change creates packages under src/google/protobuf/{io,stubs,testing} and moves build definitions there. Future changes will handle .../util and .../compiler, and finally src/google/protobuf.
These are all "toolchain-y" things, like copts, link_opts, and config_settings. These are very different from what is in //toolchain, though, so I chose the somewhat common name build_defs for the package. For now, I am only using this package for purely internal things. (Most public "defs"-type things should come from rules_proto/rules_cc, anyhow.)
This unrolls several lists based on WELL_KNOWN_PROTO maps, and move internal java codegen rules next to the corresponding java_library targets that expose them.
This avoids conflicting names in a couple of cases.
1. Within google, we want to sync files but not name them BUILD (since the structure doesn't match).
2. On case-insensitive filesystems, `build` may be used for a build directory. Naming `BUILD.bazel` avoids potentioal conflicts.
