Closes#799.
Adds standard flake outputs so modules can be accessed via:
nur.nixosModules.repos.${repo}.${module}
nur.homeModules.repos.${repo}.${module}
nur.darwinModules.repos.${repo}.${module}
nur.flakeModules.repos.${repo}.${module}
nixosModules falls back to the legacy 'modules' attribute when
'nixosModules' is not present in a repo. Missing attributes in
any repo gracefully return {}.
Amp-Thread-ID: https://ampcode.com/threads/T-019d1fc6-58ae-750f-bac6-f5e66598ddfc
Co-authored-by: Amp <amp@ampcode.com>
