Separate headers from source files

The short answer for why we need to do this is so we can consistently do
`#include "nix/..."`. Without this change, there are ways to still make
that work, but they are hacky, and they have downsides such as making it
harder to make sure headers from the wrong Nix library (e..g.
`libnixexpr` headers in `libnixutil`) aren't being used.

The C API alraedy used `nix_api_*`, so its headers are *not* put in
subdirectories accordingly.

Progress on #7876

We resisted doing this for a while because it would be annoying to not
have the header source file pairs close by / easy to change file
path/name from one to the other. But I am ameliorating that with
symlinks in the next commit.

nix-meson-build-support/export/meson.build

@@ -16,7 +16,6 @@ import('pkgconfig').generate(
   filebase : meson.project_name(),
   name : 'Nix',
   description : 'Nix Package Manager',
-  subdirs : ['nix'],
   extra_cflags : ['-std=c++2a'],
   requires : requires_public,
   requires_private : requires_private,