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.
148 files changed