Use .gitmodules to discover origins

.gitmodules is a file created at the root of a git repository when this repository has submodules. It contains URLs to other Git repositories. We could/should use it for two reasons:

1. discovery of new origins
2. completeness of the repository containing .gitmodules; as it would reference revisions of these other repositories.

Possible ways to implement it:

1. A lister scanning through SWH's DB? (ew)
2. Indexer?
3. Make the git loader create these origins?

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Migrated from T3311 (view on Phabricator)