Add support for indexing multiple repositories

Counterpart of gitlab#205178

We currently only support indexing a single repository, which prevents us from using the Elasticsearch client's bulking behavior effectively.

We want to move the indexer to run for N repositories to minimize the number of call done the the Elasticsearch Bulk Indexing API.

Implementation

• Refactor the Submitter to be project agnostic
• Support sending repositories to index using a file (or STDIN)
• Add error reporting logic by repository