Refactor `uv python install` (#8701)
Pulling out of https://github.com/astral-sh/uv/pull/8650 for
readability.
Trying to clean this up to simplify extensions in the future. This is
not a strict refactor, there are behavioral changes here.
- Adds some structs for managing state.
- Addresses some likely inconsistent behavior for weird edge-cases.
- We fill platform information before checking if a request is
satisfied.
- We error earlier if we can't find a download for the request, i.e.,
even if you somehow have it installed.
- Only reports versions as uninstalled if a download actually replaces
them.
- Moves some of the default output to tracing messages.
- Even if an installation was already satisfied, we'll check that it is
setup properly
