Ensure NXRM can proxy its own Composer repos

[fjmilens3]

The provider files for NXRM-based proxy, hosted, and group repos rely on a somewhat "flattened" form of the provider files in order to facilitate lookup and group merging.

These work with the client but are nonstandard (though not invalid) with respect to normal Composer layouts, so we should ensure that we handle this special case once #22 is finished.

As a practical matter, we have two options:

1. Add a single level of indirection (i.e. introduce a single provider-includes file to store the content currently returned in the packages.json).
2. Handle situations where the "providers" section of the packages.json is populated (as it is for Nexus repos but not for most repos in the wild).

Option (1) would make NXRM seem more consistent with other upstream providers but option (2) would make it more robust in handling any other repos that use that same approach. I suspect option (2) would be the best choice here unless we encounter a reason to change the organization of the packages.json in NXRM.