This was accomplish by moving the current overrides overlay out of
./default.nix into to a seperate file ./overrides.nix, and composing
that overlay an overlay ,`mainProgramOverrides`, (defined in
./default.nix) which uses the contents of a new file ./main-programs.nix
to create overrides that add `meta.mainProgram` to packages.
The following changes were also made to existing overrides:
* `self` and `super` where changed to `final` and `prev` respectively
* Existing additions of `mainProgram` were moved to ./main-programs.nix
* References to `pkgs.lib` were changed to `lib`
* References to `pkgs.nodejs` were changed to `nodejs`
* References to `nodePackages` were changed to `final`
* References to `pkgs.callPackage` were changed to `callPackage`
Finally `meta.mainProgram` was added to all packages that provide a
single executable whose name differs from the package's name, for
packages available on `{aarch64,x86_64}-darwin` and `x86_64-linux`.
