uv-pep508: add more routines for manipulating extras
In the course of working on #9289, I've had to devise
some additions to our markers. While we are still staying
strictly compatible with the PEP 508 format, we will be
abusing the `extra` expression to carry a lot more
information.
Specifically, we want the following additional
operations:
* Simplify `extra != 'foo'`
* Remove all extra expressions
* Remove everything except extra expressions
My work on #9289 requires all of these (which will be
in a future in PR).
