Apply rules_license to third_party tree

See; [license checking with Bazel](https://docs.google.com/document/d/1uwBuhAoBNrw8tmFs-NxlssI6VRolidGYdYqagLqHWt8/edit#) for background.

We should use rules_license in Bazel. This would
- serve as a close-to-home testbed of the licensing rules
- be a more precise way of delineating what licenses are used in bazel itself and/or the build process.

Besides being of interest to the Bazel team, it is useful to anyone who distributes their own copy of Bazel, in either source or binary form.

The first round of actual work would be
- vendor rules_license into //third_party. It would be used for bulding Bazel but should not be re-exported as a tool
- create add a top level target to produce a licenses used audit for bazel
- go through third_party and add license definitions based on the actual license text