Include Project indexer lib into the plugins API

Plugins need to be able to listen and trigger project reindexing
and thus needs to access the ProjectIndexer from the plugin library.

Change-Id: I0c6ecbbdc4d697342e5d7b0c97d66dd7697ae046

plugins/BUILD

@@ -32,6 +32,7 @@ EXPORTS = [
     "//java/com/google/gerrit/common:server",
     "//java/com/google/gerrit/extensions:api",
     "//java/com/google/gerrit/index",
+    "//java/com/google/gerrit/index/project",
     "//java/com/google/gerrit/index:query_exception",
     "//java/com/google/gerrit/lifecycle",
     "//java/com/google/gerrit/metrics",