RFE: "impure" input files/use host repositories

[evan-goode]

For https://gitlab.com/fedora/bootc/tracker/-/issues/4, we want to use a libpkgmanifest input file to declare the packages we want to layer, but we don't want the user to need to duplicate repository configuration and/or vars that are already specified in the image itself.

So we probably want either

1. a --use-host-repos option added to dnf5 manifest resolve to supplement the repositories defined in rpms.in.yaml with the repositories from DNF configuration (or ignore the rpms.in.yaml repos entirely)
2. or a way to specify in a libpkgmanifest infile that the host repositories should be used.

A more extreme approach would be to unify repository configuration around libpkgmanifest infiles for bootc-based systems, and have some kind of system for composing multiple infiles. In that case the resolve command might look like

sudo dnf5 manifest resolve --input=/usr/share/whatever/repositories.in.yaml --input=/etc/dnf/local/rpms.in.yaml

The /etc/dnf/local/rpms.in.yaml would have no repositories defined and use the repositories defined in the image-provided /usr/share/whatever/repositories.in.yaml.