Fix a flaky test relying on nondeterminsm

Fix a flaky test relying on nondeterminsm

When updating a source with multiple packages, the registry will lazily discover
both the precise and imprecise versions of the source at some point. Previously
the source would be updated depending on which was discovered first, but this
commit adds logic to understand that if an imprecise version is discovered after
a precise version then the imprecise version should be favored (because it will
always trigger an update).

This needs to understand, however, that if `cargo update --precise` is used that
those sources should not be updated again, so the sources treated in
`add_sources` are considered "special" in the sense that they're locked and will
not be updated again.