This PR adds a test replicating Kim's diamond dependency example.
The top-level package, `D`, depends on two intermediate packages, `B`
and `C`, which each require semantically different versions of another
package, `A`. The portion of `A` that `B` and `C` publicly use is
unchanged across the versions, but they both privately make use of
changed API. Currently, this causes a version clash. This will be made
to work without error later this quarter.
2b85e29cc9