Add __standalone_mode__ variable to differentiate between build modes

In some cases plugins need to differentiate the path of dependencies
in gerrit tree and standalone build mode. Provide python variable and
set it to True and False in standalone and in gerrit tree build modes.

Change-Id: Iad1f899e8d55a4d59e21bddc0bc77b5599ec14b4
diff --git a/gerrit_plugin.bucklet b/gerrit_plugin.bucklet
index 507955c..4d73d0d 100644
--- a/gerrit_plugin.bucklet
+++ b/gerrit_plugin.bucklet
@@ -53,6 +53,8 @@
   '//lib/gwt:dev',
 ]
 
+__standalone_mode__ = True
+
 def gerrit_plugin(
     name,
     deps = [],