Support @extend in the module migrator

[jathak]

The migrator should add additional @use rules to ensure that selectors being extended are upstream of the extending selector.

An internal team ran into this issue when migrating. The compiler saved them for extended placeholder selectors, but not for a concrete selector that was also present within the same stylesheet as the @extend (which they didn't notice until their screenshot tests failed).

[jathak]

@nex3: Do you know what the right way to go about doing this would be? Would having References track extensions make sense?

[nex3]

That sounds like a reasonable place to start, yeah.