In dissolving top level directory series we lost this source package
and as the consequence, GWT enabled gerrit plugins cannot be built in
standalone build mode.
Test Plan:
* Build the gerrit-plugin-gwtui artifact:
$ bazel build gerrit-plugin-gwtui/gwtui-api_deploy.jar
* Verify that the sources for extensions package are included:
$ unzip -t bazel-bin/gerrit-plugin-gwtui/gwtui-api_deploy.jar | grep AccountFieldName.java
testing: com/google/gerrit/extensions/client/AccountFieldName.java OK
Change-Id: I3c80723b470b5ccf96ecd7c9edb75dadc0630d75