Support `mainProgram` as an attribute of `meta` for packages.
This is an attribute used by [`nix
run`](https://nixos.org/manual/nix/unstable/command-ref/new-cli/nix3-run.html#description)
to customize the main program of a package.
For example, `pkgs.neovim` provides a `/bin/nvim` executable which users
would (almost certainly) prefer `nix run` to execute instead of a
non-existing `/bin/neovim`.
Signed-off-by: Ana Hobden <operator@hoverbear.org>
acfddd576e