Don't implement allow-newer/allow-older by modifying PackageDescription

It seems cleaner to hook it straight into the dep solver, rather than try to ninja it in by package description modification. If you do it by modification, you have to worry about making sure the "correct" package description is used anywhere you want to look at deps, and sometimes you get spurious warnings because it looks like you modify build-depends which affects our source level checks, but actually we didn't.

The lack of cleanliness here is hurting me because I want BuildInfo to reflect the written syntax as closely as possible (when I make the modification for #4155), but I have to keep `targetBuildDepends` around as a real field if people are editing it.

Previously @kosmikus requested this in #3466.