remote-ls: Rename and modify helper struct

This commit changes the struct "RemoteDirPair" to "RemoteStateDirPair"
so that an upcoming commit can make another struct called
"RemoteDirPair" in flatpak-builtins-utils.[ch] which will just have the
remote name and dir without a state. This commit also removes the
"remote_name" member, because it makes more sense to use the remote_name
field in the state object.

Closes: #2288
Approved by: matthiasclasen
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