Implement per variant package settings

[shubhbapna]

We need to be able to define per variant package settings. This will probably be a complete revamp of the settings file and we will be able to remove the prebuilt field and only have <package_name> fields

[shubhbapna]

Option 1:

Take advantage of yaml anchors (no new functionality on fromager side essentially)

torch:  &torch-base
       build_dir: ...
       resolver_dist: ...
       prebuilt: true

torch-<variant>:
  << : *torch-base
  # override whatever we want
  build_dir: ...
  pre_built: true

[dhellmann]

We just landed a change to support package-specific settings files. We could extend that to include a variant directory in the path.

• settings/package.yaml
• settings/$variant/$package.yaml

[shubhbapna]

I think #323 covers what we need for this, closing