reject-unconstrained-dependencies and qualified solving

#5475 has some discussion about `reject-unconstrained-dependencies` and the interaction with qualified goals.

There are at least two options here, and they aren't necessarily mutually exclusive:

* a new mode that will only restrict unconstrained and unqualified dependencies; qualified goals are free to pick any version

* or being able to separately restrict each qualified goal (e.g., being able to nail down the version `happy:Setup.Cabal` separately from `cpphs`).

This probably isn't worthwhile until #3502 is solved.

Start by reading the discussion in #5475 and the prerequisite work in #3502. Determine which dependency restrictions are intended for unconstrained, unqualified, and qualified goals, then confirm that the chosen behavior is specified clearly enough to implement and test.

Written by the indexing model from the issue text.