Default loading of prezto modules breaks prezto docs

[alextes]

The ZGEN_PREZTO_LOAD_DEFAULT is pretty nice, still, it took me a while to figure out that the reason why prezto wasn't coloring output had to do with zgen loading modules by default. Prezto looks for dircolors which on macOS is gdircolors, prezto warns you about, and suggests you use the gnu-utility module with the option: zstyle ':prezto:module:gnu-utility' 'prefix' 'g', and load the module before the utility module. Even though I did, after some debugging I found out utility was still loading before gnu-utility. The reason? zgen adds default modules which include utility and prezto skips utilities that are already loaded.

Since I imagine most macOS users would like ls coloring, and most macOS users don't force brew to replace core utils with GNUs versions for potential system instability problem, I think zgen's default behavior is problematic here.

How about we make default modules off by default? That would solve this issue entirely I imagine.

Related to #112

Thanks for an awesome utility 🙏 !