The main purpose of this PR is to make the basis for
`mkSkeletonFromList`'s decision between `cpu-kernel-libcabi` vs
`cpu-vendor-os` clear, without changing its behavior. The existing
code obscures this decision behind a sequence of prioritized matches
(i.e. `if-then`) which jump around between different coordinates.
Two side benefits of this PR:
1. It makes the root cause of #165836 obvious: we are missing a case
for `cpu-vendor-libcabi`. This is why nixpkgs stumbles over
`*-none-*`.
2. It illuminates some very weird corner cases in the existing
logic, like `*-${vendor}-ghcjs` overriding the `vendor` field,
and `mingw32` being transformed into `windows` in some cases.
Co-authored-by: John Ericson <git@JohnEricson.me>
It gives a warning on the lazy-trees branch of Nix
(NixOS/nix#6530)
one of these was also giving me an error (the one in lib/trivial probably)
```
$ nix build
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/installer/sd-card/sd
-image-aarch64.nix' and then accessing it is deprecated, at /home/artturin/nixgits/my-nixpkgs/lib/modules.
nix:349:99
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/.git' and then accessing it is dep
recated, at /home/artturin/nixgits/my-nixpkgs/lib/sources.nix:35:32
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/system/etc/etc.nix'
and then accessing it is deprecated, at «stdin»:0
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/system/etc/etc-activ
ation.nix' and then accessing it is deprecated, at «stdin»:0
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/installer/sd-card/sd
-image-aarch64.nix' and then accessing it is deprecated, at «stdin»:0
error: cannot decode virtual path '/nix/store/virtual0000000000000000000000005-source'
(use '--show-trace' to show detailed location information)
```
A tricky thing about FreeBSD is that there is no stable ABI across
versions. That means that putting in the version as part of the config
string is paramount.
We have a parsed represenation that separates name versus version to
accomplish this. We include FreeBSD versions 12 and 13 to demonstrate
how it works.
