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/libraries/java/jflex/default.nix

@@ -32,6 +32,7 @@ stdenv.mkDerivation rec {
   meta = {
     homepage = "https://www.jflex.de/";
     description = "Lexical analyzer generator for Java, written in Java";
+    mainProgram = "jflex";
     license = lib.licenses.bsd3;
     platforms = lib.platforms.unix;
   };