Refactor of exports mixin

The logic of the `exports()` mixin has been refactored into two functions:
  - `using()`, which checks if a component is on the list (`$import-css`) of components that should be imported
  - `imported()`, which checks if a component has already been imported (`$exports`)

The `using()` function is handy in other contexts; it allows you to check within one component's Sass if another component is being used, and if it is, export CSS that applies to both.

The mixin doesn't work perfectly yet; in some cases components still import multiple times.