Bazel: Export core server/schema library in Plugin API
Bazel: Export core server/schema library in Plugin API

In I60033ef0fa server/schema package was split off from the server
package, but the new rule wasn't exported in the Plugin API.

Some plugins, most notably verify-status plugin, depend on that.

Change-Id: Ie1cdf246566c60ea6d23e59246e34818f9667c6d

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