treewide: add meta.mainProgram to packages with a single binary

The nixpkgs-unstable channel's programs.sqlite was used to identify
packages producing exactly one binary, and these automatically added
to their package definitions wherever possible.

pkgs/development/interpreters/gpython/default.nix

@@ -39,6 +39,7 @@ buildGoModule rec {
 
   meta = with lib; {
     description = "A Python interpreter written in Go";
+    mainProgram = "gpython";
     homepage = "https://github.com/go-python/gpython";
     changelog = "https://github.com/go-python/gpython/releases/tag/${src.rev}";
     license = licenses.bsd3;