Properly validate configured collection IDs

This commit is a follow-up for the commit "dir: Validate locally
configured collection IDs". Whereas in that commit we validate the
collection ID in flatpak_remote_state_fetch_commit_object(), here we do
it in _flatpak_dir_get_remote_state(), since fetch_commit_object() is
not called on the code path normally taken for a transaction (only if
try_resolve_op_from_metadata() fails and in other niche scenarios).

This should ensure that if someone erroneously sets a collection ID on a
remote, the error will be caught quickly. It also helps the eos-updater
unit tests pass.
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