Merge branch 'stable-3.1'

* stable-3.1:
  Upgrade bazlets to latest stable-3.1 to build with 3.1.6 API
  Move external dependencies to external_plugin_deps.bzl
  Upgrade bazlets to latest stable-3.0 to build with 3.0.10 API
  Upgrade bazlets to latest stable-2.16 to build with 2.16.21 API
  Upgrade bazlets to latest stable-2.15 to build with 2.15.19 API
  Convert File.NameKey to AutoValue
  Upgrade bazlets to latest stable-2.16 to build with 2.16.20 API

Change-Id: Ib314440288b220abaf33b0884bf19f6a10798d83