fix(optimize-css): indexing should account for nested components

[metonym]

The optimize CSS plugin uses a pre-computed index of Carbon components (exported from the barrel file) to determine what classes to prune.

However, most components compose other components. These sub-components (are their sub-components) must be accounted for.

An example would be a selectable DataTable:

<DataTable selectable />

The checkboxes for DataTable are a bespoke, internal-facing component. The optimize plugin does not include the bespoke checkbox styles.

The solution is to:

• Record a list of sub-component references when extracting selectors from a component.
• If a component has a list of sub-components, append the additional selectors after all files have been processed.