Implement custom resolution failure reporter to hide root package versions (#300)
Extends #295
Closes #214
Copies some of the implementations from `pubgrub::report` so we can
implement Puffin `PubGrubPackage` specific display when explaining
failed resolutions.
Here, we just drop the dummy version number if it's a
`PubGrubPackage::Root` package. In the future, we can further customize
reporting.