Merge branch 'stable-3.0' into stable-3.1

* stable-3.0:
  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

Adapt the moved auto-value version to the one used in this branch.

Change-Id: Icd412164371152ff4ec2624bed0d94df8bd8a277