Bug with some `@extend`s defined in a module that is then imported.

[jathak]

@extend doesn't work properly in certain cases that mix @import and the module system.

I would expect

// entrypoint.scss
@import "library";

// _library.scss
@forward "concrete";

// _placeholder.scss
%foo {
  .bar {
    color: blue;
  }
}

// _concrete.scss
@use "placeholder";

.foo {
  @extend %foo;
}

to compile to:

.foo .bar {
  color: blue;
}

but instead I get no output.

It works fine if entrypoint instead @uses library, and it also works fine if %foo is on its own without .bar.