This commit makes
flatpak_installation_list_remote_refs_sync() work for local URIs
pointing at sideload repos, e.g.
"file:///media/mwleeds/flatpaks/.ostree/repo" where a command "flatpak
create-usb /media/mwleeds/flatpaks ..." was previously run. This is
important because the Endless fork of gnome-software uses that API to
list apps on a USB drive. The API worked before Flatpak 1.7.1 so making
it work again basically involves reverting parts of
336a127f55.
This commit also makes the remote-ls command work for file:// URIs to
sideload repos, which is useful behavior for debugging, and is also the
behavior mentioned in this blog post:
https://blogs.gnome.org/mclasen/2018/08/26/about-flatpak-installations/
367a3f3efa