This is needed to avoid confusing and repeated boilerplate for
`fooForTarget`.  The vast majority of use-cases can still use
`buildPackages or `targetPackages`, which are now defined in terms of
these.

`pkgsBuildTarget` allows us to avoid repeated and confusing conditions.
The others merely provide clarity for one the foreign package set's
target platform matters.

There was a bunch of stuff in the cross section that haddn't had any
attention in a while. I might need to slim it down later, but this is
good for now.

Thanks @matthewbauer!

Co-Authored-By: Ericson2314 <git@JohnEricson.me>

top-level: Create `pkgs{Build,Host,Target}{Build,Host,Target}`