Add a "Process only co-located styles" flag?

[lougreenwood]

It would be useful if it were possible to tell e-c-m to only process files which come from co-location.

This would provide a useful migration path away from ember-component-css, which has issues with Glimmer Components. The upgrade path would look something like:

• Whilst ember-component-css is installed
• Install e-c-m
• Enable "co-location only" flag
• Migrate a component to GC and migrate it's css to css-modules - as a co-located component.

I think that this would simplify the workaround proposed by @knownasilya here: https://ilyaradchenko.com/running-component-css-and-css-modules-in-the-same-app/