Solver: Combine "skipped" versions with previous "rejected" version when possible

Here is the error message that is currently produced by cabal when a package has an unsatisfiable dependency like in #4251. (I modified `cabal-testsuite.cabal` to require a later version for `aeson`.)

```
Resolving dependencies...
Error: [Cabal-7107]
Could not resolve dependencies:
[__0] trying: cabal-testsuite-3 (user goal)
[__1] next goal: aeson (dependency of cabal-testsuite)
[__1] rejecting: aeson-2.2.3.0 (conflict: cabal-testsuite => aeson>=3.0 && <3.1)
[__1] skipping: aeson; 2.2.2.0, 2.2.1.0, 2.2.0.0, 2.1.2.1, 2.1.2.0, 2.1.1.0, 2.1.0.0, 2.0.3.0, 2.0.2.0, 2.0.1.0, 2.0.0.0, 1.5.6.0, 1.5.5.1, 1.5.5.0, 1.5.4.1, 1.5.4.0, 1.5.3.0, 1.5.2.0, 1.5.1.0, 1.5.0.0, 1.4.7.1, 1.4.7.0, 1.4.6.0, 1.4.5.0, 1.4.4.0, 1.4.3.0, 1.4.2.0, 1.4.1.0, 1.4.0.0, 1.3.1.1, 1.3.1.0, 1.3.0.0, 1.2.4.0, 1.2.3.0, 1.2.2.0, 1.2.1.0, 1.2.0.0, 1.1.2.0, 1.1.1.0, 1.1.0.0, 1.0.2.1, 1.0.2.0, 1.0.1.0, 1.0.0.0, 0.11.3.0, 0.11.2.1, 0.11.2.0, 0.11.1.4, 0.11.1.3, 0.11.1.2, 0.11.1.1, 0.11.1.0, 0.11.0.0, 0.9.0.1, 0.9.0.0, 0.8.1.1, 0.8.1.0, 0.8.0.2, 0.7.0.6, 0.7.0.4, 0.6.2.1, 0.6.2.0, 0.6.1.0, 0.6.0.2, 0.6.0.1, 0.6.0.0, 0.5.0.0, 0.4.0.1, 0.4.0.0, 0.3.2.14, 0.3.2.13, 0.3.2.12, 0.3.2.11, 0.3.2.10, 0.3.2.9, 0.3.2.8, 0.3.2.7, 0.3.2.6, 0.3.2.5, 0.3.2.4, 0.3.2.3, 0.3.2.2, 0.3.2.1, 0.3.2.0, 0.3.1.1, 0.3.1.0, 0.3.0.0, 0.2.0.0, 0.1.0.0, 0.10.0.0, 0.8.0.1, 0.8.0.0, 0.7.0.5, 0.7.0.3, 0.7.0.2, 0.7.0.1, 0.7.0.0 (has the same characteristics that caused the previous version to fail: excluded by constraint '>=3.0 && <3.1' from 'cabal-testsuite')
[__1] fail (backjumping, conflict set: aeson, cabal-testsuite)
After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: cabal-testsuite, aeson
```

I think that it would be clearer to combine all of the versions after "skipping" with the previous "rejecting" message, since they describe the same conflict in this case. Combining the lines would also allow us to summarize the versions more effectively, as in #9831.

Reproduce the dependency-resolution error by modifying cabal-testsuite.cabal to require an unavailable aeson range, then inspect the dependency solver's diagnostic path. Compare the requested consolidation with the summarization discussed in #9831; done means skipped versions sharing the rejection cause are combined into a clearer, summarized message.

Written by the indexing model from the issue text.