The Bazel rules for Go prefix generated source files with a label that the
compiler expects to find. Ensure we don't remove that label and thus confuse
Also: Handle packages with no source inputs gracefully in the indexer.
A package with no source files is usually a sign of a bug, typically in the
extractor. The indexer now reports an error rather than crashing, when this
Reviewers: #core_team, shahms
Reviewed By: #core_team, shahms
Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1272