Redirected projects should be consolidated

It looks like if a repo has ever been moved from org to org, all of its historical names show up in the data, even if the old org no longer exists, or if the old URLs now redirect to the newest organization.

Examples:

Spack was first `scalability-llnl/spack`, then `LLNL/spack`, then `spack/spack`. All three appear even though only `spack/spack` exists, and the other two redirect to `spack/spack`, and `scalability-llnl` no longer exists:

The `eaysbuilders/easybuild[-*]` repos used to be in the `hpcugent` org. Now they're not, but the old org still shows up in the data:

I think nodes for repos that now redirect to new locations should be consolidated into the node of the new location.

No files or tests are named. Start by tracing how repository and organization nodes are generated and how GitHub redirects are represented, using the Spack and EasyBuild examples as fixtures. Done means repositories that redirect to a current location are consolidated and obsolete organizations no longer appear in the data.

Written by the indexing model from the issue text.