Bazel: switch BUILD to native ext_deps support

Use native `ext_deps` support in `gerrit_plugin()` and
`gerrit_plugin_tests()` in the plugin BUILD file.

Declare external dependencies as Maven coordinates instead of generated
Bazel labels, and share common dependency through `EXT_DEPS`.

Change-Id: Ib35bcd7a84e64f6421fd9e2876020ccc0839a123