shared/conf-parser: collapse pkgdir and conf_file args into one

shared/conf-parser: collapse pkgdir and conf_file args into one

This essentially reverts 5656cdfeeabc16b5489f5ec7a0a36025a2ec1f23. I find it
much easier to understand what is going on when the
path-relative-to-the-search-path is passed in full, instead of being constructed
from two parts, with one of the parts being implicit in some places.

Also, we call 'systemd-analyze cat-config <path>' with <path> with the same
meaning, so this makes the internal and external APIs more consistent.