core: pkgrepo: support arbitrarily named pmaports directories (MR 2470)

There was an oversight when this API was originally created and it
implicitly assumed that the pmaports repository was always named
"pmaports". This unfortunately broke some peoples workflows.

Introduce a new "pkgrepo_name()" function and adjust the codebase to use
it, as well as adjusting pkgrepo internally to special case the
"pmaports" repo so that it's always named pmaports no matter what the
directory itself is named.

This is probably more complexity than we should be dealing with here, we
should probably create a new type to encode this behaviour.

Fixes: #2412
Signed-off-by: Caleb Connolly <caleb@postmarketos.org>