Move libraries to internal directory (#66) * Moved all libraries to the internal subdirectory and set visibility so that only other Gazelle libraries may depend on them. These libraries don't have a stable interface and should not be depended on outside of Gazelle. * Moved testdata to internal/rules/testdata. It's only used by that package. * Moved rules/internal/gen_std_package_list to internal/rules/gen_std_package_list.