solver: invert heuristics for choosing the next dependency to resolve so that...

solver: invert heuristics for choosing the next dependency to resolve so that dependencies with more versions are resolved first (#8256)

Although the original algorithm proposes to choose dependencies with fewer versions first, we don't know about any real-world examples where this is important. However, we know about real-world examples (especially boto3 vs. urllib3, but also Sphinx vs. docutils) where it's better to choose dependencies with more versions first.

Co-authored-by: David Hotham <david.hotham@blueyonder.co.uk>