Allow "mutating" the namespaces of mappings #1320
Description
Basically allow Loom to change namespaces of specific sets of mappings.
An example use case of this, together with #1297, is to support multiple intermediary targets at one time, for instance in legacy versions where you have more than one option of intermediaries like Ornithe and LF, by first remapping to official then to the corresponding intermediary namespaces.
EDIT: Okay I'm dumb. You can simply choose one intermediary as the major target and place other intermediaries under different namespaces as mapping layers so that you can directly remap the src to them