Support --forward=prefixed across migrator runs

[jathak]

Fixes #123.

If a member was unprefixed by a previous migrator run, running the migrator on a downstream stylesheet with --forward=prefixed should still forward that member.

[jathak]

Added a test with a different prefix. --forward=prefixed is not allowed without --remove-prefix, so I don't think a test is necessary for that.

[jathak]

Oh, yeah, I forgot about cases like that. Would this be the best output?

<==> output/entrypoint.scss
@use "button";

@forward "button" as button-*;

a {
  color: button.$variable;
}

<==> output/entrypoint.import.scss
@forward "entrypoint" as lib-*;

[nex3]

I think so, yeah.

[jathak]

Okay, this should now support the subprefix case.