Inconsistent structure between fabric and forge subprojects, mixin rules don't apply to platform specific code

[mrmangohands]

Bug Description

Currently fabric specific mixins are located under the sodium.mixin.fabric package while neoforge ones are under sodium.neoforge.mixin. Neither of these layouts let mixin rules from common apply to mixins in the subprojects. Either the mixin rule system would need to be updated to apply automatically to matching subpackages in the subprojects, or both could just use sodium.mixin as the base, with a neoforge, fabric, or generically platform bottom level subpackage. So for example instead of sodium.mixin.fabric.core.WindowMixin you could have sodium.mixin.core.fabric.WindowMixin.

We should decide which of the 3 layouts we'd like to go with. The first and third options can be combined if desired. Packages that have a match in common or the other platform could use a bottom level platform package, and either everything else or just the Mixins that provide platform specific APIs could use the top level one.

[mrmangohands]

Edited for clarity