Improve error messaging when a dependency is not found (#1241)
Previously, whenever we encountered a missing package we would throw an
error without information about why the package was requested. This
meant that if a transitive dependency required a missing package, the
user would have no idea why it was even selected. Here, we track
`NotFound` and `NoIndex` errors as `NoVersions` incompatibilities with
an attached reason. Improves our test coverage for `--no-index` without
`--find-links`.
The
[snapshots](https://github.com/astral-sh/puffin/pull/1241/files#diff-3eea1658f165476252f1f061d0aa9f915aabdceafac21611cdf45019447f60ec)
show a nice improvement.
I think this will also enable backtracking to another version if some
version of transitive dependency has a missing dependency. I'll write a
scenario for that next.
Requires https://github.com/zanieb/pubgrub/pull/22
