Add support for external repositories

The lint rule hardcoded `@//` which meant that it could only come form the top-level repository.

I'm building a third party (external) repo with lint configs and it needs to reference the rule from the appropriate external repo.

lint/defs.bzl

@@ -33,7 +33,7 @@ def get_lint_config(linter_name, tags):
         return None
 
     if native.existing_rule("%s_%s" % (OVERRIDE_RULE_NAME, linter_name)) != None:
-        return Label("@//%s:%s_%s" % (native.package_name(), OVERRIDE_RULE_NAME, linter_name))
+        return Label("@%s//%s:%s_%s" % (native.repo_name(), native.package_name(), OVERRIDE_RULE_NAME, linter_name))
 
     if linter_name in configured_linters:
         return Label("@apple_linters//:%s_%s" % (OVERRIDE_RULE_NAME, linter_name))