remote-ls: Allow a URI instead of remote name

This fixes the ability of the remote-ls command to take a file:// URI
instead of a remote name, which is especially useful for repos on USB
drives (created via `ostree create-usb`) which are temporary and don't
warrant being added to the repo config. This commit also updates
relevant documentation, adds a unit test, and updates a few variable
names to improve readability.

I can't find a commit in the history where this was working, but it's
working on the Endless fork of flatpak so I think there was agreement at
some point that it's desired behavior.

Fixes https://github.com/flatpak/flatpak/issues/1588

Closes: #1587
Approved by: mwleeds
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