Typo that was introduced in Ia6cf1bad wasn't discovered for years.
When plugin is trying to add classpath rule:
  java_library(
    name = 'classpath',
    deps = GERRIT_GWT_API + GERRIT_PLUGIN_API + [
      ':<plugin>__plugin',
    ],
  )
then Buck is reporting non-existing rule. Note that in bucklets
the correct constant is defined:
  GERRIT_GWT_API = ['//lib/gerrit:gwtui-api']
Change-Id: Ice8418d27b08a8ee7925a4040c3a18ce5cd553f2