Add a path lookup/resolver

[xparq]

• applying the same...? or simpler? (is_abs-based?)... prefixing as in prefix_by_intent to a list of prefix paths,
• with also (then necessarily...) checking if the resulting path actually exists.
• ...but then: the feature of constructing a prefixed path regardless of existing or not (i.e. output paths) should still be preserved (obviously for the single-prefix-path flavor of this API)!