Bazel: Add extensions sources to gerrit-plugin-gwtui artifact
Bazel: Add extensions sources to gerrit-plugin-gwtui artifact

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

WANdisco/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit