Implement Bazel build

This change adds two Bazel build modes:

1. Standalone build mode (Bazlets driven)
2. In Gerrit tree build mode

Bazlets repository is currently consumed from GH, but can be moved
later to gerrit-review.

TEST PLAN:

  1. run `bazel build reviewers` in reviewers project
  2. link reviewers directory to gerrit/plugins and run
     `bazel build plugins/reviewers` in gerrit tree

Change-Id: Ie60e32096a039f75402a202ff14556118b18d86f
8 files changed