Migrate to bazelmod

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

* gerrit_plugin: use plugin= instead of name=

Change-Id: I82b4608bd281d35d6cc2730827a4d01e3a12e091