Merge branch 'stable-2.15'

* stable-2.15:
  Update bazlets to the latest on stable-2.14 to use 2.14.7 release API
  Revert "Abstract external_plugin_deps() one level"
  Abstract external_plugin_deps() one level

Change-Id: Ib4f190ac52055103c449f6f6d774d6e83f35cd2f