`allow-newer: latest:PKG` (and `constraint: PKG latest`)

Preparing for new GHCs we often need to test our package with dependencies whose upper bounds have not been bumped yet.

A bare `allow-newer: *:DEPS` is (with e.g. DEPS the GHC-preinstalled packages) often ruins the build, bringing tons of outdated packages into the plan. (E.g. there could be a stone-age version of `foo`, say `foo-0.2`, kept in check by `base < 4.5` that would suddenly be unleashed onto us.)

I would find `allow-newer: latest:DEPS` more useful. It would allow bumping `DEPS` in only the _latest_ available version. E.g. if this was `foo-3.1.4`, then it would enjoy relaxed upper bounds for `DEPS`, but `foo-0.2` would still kept outside of the party.

This is hard to work around, because it is not easy to get the transitive dependencies of a package without having a valid build plan in the first place.

(Along similar lines, I would sometime find a `constraint: DEP latest` useful to demand the latest version of package DEP. This is easier to work around. But maybe this is also easier to implement.)

In anticipation that we might get `allow-newer: latest:...` at some point, I squatted the name `latest` on Hackage so that it does not accidentially get used. (Such a package would conflict with the proposed syntax, as you can e.g. have `allow-newer: mypackage:base`.)

Lifted from:
- https://github.com/haskell-CI/haskell-ci/issues/642#issuecomment-1437507237