The extension package exposes some sources to the client. Make them
available to the plugins as well. To use, the plugin only needs to
inherit the new Extensions GWT module:
<inherits name="com.google.gerrit.extensions.Extensions"/>
Given that the class files and javadocs for exposed sources are already
contained in the gerrit-plugin artifact, we don't expose them again in
the plugin-gwtui artifact.
Change-Id: I4b98ee470614a5eab1bd96cc7801895336851cc1
9e218ae090