Modernize BUILD to bazlets gerrit_plugin

Load gerrit_plugin from bazlets instead of the in-tree
tools/bzl:plugin.bzl, matching the other externally hosted plugins.
The bazlets macro works both in-tree, via the in_gerrit_tree flag,
and in a standalone build.

Change-Id: I1f7e6b58c2d90a34e5f8a17d94c3b60712e5d8f3