backport "[QP] Correctly find joined fields in the previous stage (#40933)" (#40987)

This logic was trying to match `[name join-alias]` pairs previously, but
that won't work since the later stage field doesn't have a `:join-alias`
on it. This falls back to matching on only the ID or name, but only if
it's unambiguous.

See #40252.

Co-authored-by: Braden Shepherdson <braden@metabase.com>