Add failing test for pre-release and backtracking behavior

[mpizenberg]

Hi, I have been spending a lot of time on a Rust port of pubgrub and stumbled upon a tricky case regarding handling of pre-releases and backtracking behavior. In particular, due to the creation of NoVersions incompatibilities potentially preventing the selection of a pre-release version after backtracking.

So I've checked how pub handles this by adding a test and it seems that pub fails to solve dependencies in that situation. Here is the test I've added:

  test('unable to backtrack to pre-release after NoVersions', () async {
    await servePackages((builder) {
      builder.serve('a', '1.1.0', deps: {'b': '^1.0.0'});
      builder.serve('b', '1.0.0', deps: {'c': '^1.0.0'});
      builder.serve('c', '0.9.0');
      builder.serve('b', '1.1.0-alpha');
      builder.serve('a', '1.0.0', deps: {'b': '^1.1.0-alpha'});
    });

    await d.appDir({
      'a': '^1.0.0',
    }).create();
    await expectResolves();
    // fails though I would have expected it to succeed with either
    // a 1.0.0, b 1.1.0-alpha or
    // a 1.1.0, b 1.1.0-alpha
  });

It consists of putting the solver on the wrong track by making it pick a 1.1.0 followed by b 1.0.0 then realizing that there is no compatible c. At that moment, it should backtrack and remember the wrong decision with NoVersions incompatibilities. The issue is that it seems to create knowledge preventing it to pick the b 1.1.0-alpha pre-release while it's a totally valid version after backtracking.

What are your thoughts on this?

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[sigurdm]

I included this test in: #3078