Update BUILD files to work with gazelle

[ribrdb]

Fix #200
This follows suggestions from @jayconrod:

• Add BUILD.bazel to the repository root so that gazelle will skip generating BUILD files
• Add BUILD.bazel files in gen-go with aliases to the targets in src. These are the targets gazelle will use when it runs in repositories that depend on opencensus-proto.

This required moving the WORKSPACE file to the root instead of in src, and corresponding changes to the dependencies.