stage: refactor extraPkgs

I have renamed the overlay to “otherPackageSets” because I think that
is more descriptive.

pkgsLocal has been removed because there were concerns that it would
be confusing. None of the other names seemed very useful so I think it
is best to avoid it altogether. pkgsCross is still included,
hopefully, that will not have as much confusion.

pkgsMusl is now available for building Musl packages. It will give you
packages bulit with the Musl libc.

Also added more documentation.

/cc @dezgeg @Ericson2314 @dtzWill