Wasi P2 is different enough to Wasi P1 to warrant being treated entirely
separately, rather than as two minor variants of the same thing. P3 will
likewise want to be a different target.
I've left aliases in place; maybe eventually, those can be deprecated
and removed. I've tested this, but it's possible there might be breakage
somewhere (e.g., the canonical doubles for P1 have changed, though I
can't imagine why anyone would rely on that).
Fixes https://github.com/NixOS/nixpkgs/issues/435954
Sam Pointon
629b4ee093