Migrate to bazelmod

Switch the BUILD from in-tree macros (//tools/bzl) to the bazlets
gerrit_plugin.bzl macros, which auto-detect the build mode and work
both in-tree and standalone.

* gerrit_plugin: use plugin= instead of name=
* fold junit_tests into gerrit_plugin_tests, which auto-wires the
  plugin classes, Gerrit API and acceptance framework deps

Change-Id: I32c77cfa0f7cd5139dbef9e04584e5c4ada111e9