Refactor unavailable metadata to shrink the resolver (#9769)
The resolver methods are already too large and complex, especially
`choose_version*`, so i wanted to shrink and simplify them a bit before
adding new methods to them.
I've split `MetadataResponse` into three variants: success, non-fatal
error (reported through pubgrub), fatal error (reported as error trace).
The resulting non-fatal `MetadataUnavailable` type is equivalent to the
`IncompletePackage` type, so they are now merged. (`UnavailableVersion`
is a bit different since, besides the extra `IncompatibleDist` variant,
it have no error source attached). This shows that the missing metadata
variant was unused, which I removed.
Tagging as error messages for the logging format changes.
