This commit integrates the identity plugin, which was created in #1384.
It does this by adding explicit identity fields to the project
configuration, which are then applied when loading the graph in
`api/load.js`.
The actual integration is quite straightforward.
Test plan: The underlying logic is thoroughly tested; I added one new
test case to verify that it is integrated properly. Since the project
compat has changed, I've updated all the snapshots. Prior to merging
this PR, I will produce one "integration test", using this code to do
identity resolution for a real project (i.e. on the SourceCred instance
itself).
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