lack of partial signature remapping

[tildejustin]

In prior versions of loom, or withuseLegacyMixinAp = true, signatures which did not exist in the minecraft version being compiled against could still be written partially with named, partially with intermediary mappings. Example: https://github.com/RedLime/SleepBackground/blob/31fcebfba49dd36d9edfa2d53981febcbcecaef8/src/main/java/com/redlimerl/sleepbackground/mixin/MixinMinecraftClient.java#L41-L43. With useLegacyMixinAp = false, one has to manually remap the entire signature if it does not exist in the compile target jar, since the named parts of the signature are not remapped (I verified this with vineflower).