alsa-project: expose the scope as a top-level package to support overrides

this enables overrides of the following form:
```
pkgs.extend (_self: super: {
alsa-project = super.alsa-project.overrideScope' (_alsaSelf: alsaSuper: {
alsa-ucm-conf = alsaSuper.alsa-ucm-conf.overrideAttrs (_: {
src = ...
});
});
)
```

pkgs/top-level/all-packages.nix

···
 
   tinyalsa = callPackage ../os-specific/linux/tinyalsa { };
 
-  inherit (callPackage ../os-specific/linux/alsa-project { })
+  alsa-project = callPackage ../os-specific/linux/alsa-project { };
+  inherit (alsa-project)
     alsa-firmware
     alsa-lib
     alsa-oss