treewide: Change URLs to HTTPS

Lots of URLs were HTTP redirect to HTTPS. Changed those and checked them
if there's actual content. Inspired by
https://github.com/NixOS/nixpkgs/issues/60004

pkgs/development/libraries/libffi/default.nix

@@ -88,7 +88,7 @@ stdenv.mkDerivation rec {
       conversions for values passed between the two languages.
     '';
     homepage = http://sourceware.org/libffi/;
-    # See http://github.com/atgreen/libffi/blob/master/LICENSE .
+    # See https://github.com/atgreen/libffi/blob/master/LICENSE .
     license = licenses.free;
     maintainers = [ ];
     platforms = platforms.all;