Refactor rustc_on_unimplemented's filter parser

[mejrs]

Followup to #139091; I plan on moving most of this code into rustc_attr_parsing at some point, but want to land this separately first.

I have taken care to preserve the original behavior as much as I could:

• All but one of the new error variants are replacements for the ones originally emitted by the cfg parsing machinery; so these errors are not "new".
• the InvalidFlag variant is new, this PR turns this (from being ignored and silently doing nothing) into an error:

  #[rustc_on_unimplemented(on(something, message = "y"))]
  //~^ ERROR invalid boolean flag
  //~^^ NOTE expected one of `crate_local`, `direct` or `from_desugaring`, not `something`
  trait InvalidFlag {}

  This does not occur anywhere except in this test. I couldn't find a way that I liked to keep allowing this or to do nothing, erroring was the cleanest solution.
• There are a bunch of FIXME throughout this and the previous PR, I plan on addressing those in follow up prs..

Finally, this gets rid of the "longest" dependency in rustc:

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These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
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