Add filtering for packages, not just tests

[sunshowers]

Add filtering for packages (-p can be overloaded to mean this).

This would accept the package, deps and rdeps functions, but not test. The point of this would be to possibly build a smaller set of tests (at the cost of a different feature unification result, plus the inability to reuse builds).

This doesn't block the full release of filter expressions.

[Firestar99]

If a filterset always excludes a particular binary, it will not be run, even to get the list of tests within it. This means that a command like:
cargo nextest list -E 'platform(host)'
will not execute any test binaries built for the target platform.

https://nexte.st/docs/filtersets/reference/#basic-predicates

I was expecting it to not just not run it, but also not build it as well. But it seems like it builds every single package it then just not run them, which is slowing down CI quite a bit for rust-gpu. For now, I'll go back to --exclude'ing all the crates we don't want to test on that particular CI job, but a solution within nextest would be greatly appreciated, since that would also speed up compiling tests on dev machines (assuming they don't have it compiled already).

Background: we have normal tests, difftests (verifying that multiple tests emit the same output) and compiletests (check if something compiles, potentially disassemble it and potentially verify the output against a know good, not yet ported to nextest) and want to run each one as a different job, since difftests and compiletests take a while and need to compile quite a few extra dependencies.