refactor: Add index stylesheets to each package

[nex3]

As discussed with @abhiomkar, this adds an _index.scss file to each package so that each package can be loaded with a single @use rule. As a bonus, that @use rule will automatically use the package's name as its namespace.

Here's an example of the difference between an API using each separate files versus index files.

[abhiomkar]

Sent first round of review comments.

• Can you make a CL and export into PR (See http://go/mdc-web-g3sync)? You wouldn't have to do any additional changes on GitHub except exporting a CL. (Internally we've good test coverage including screenshot tests and global tests).
• Can we also update actual _mixin, _variables files to use these index files, using index files greatly improves readability of our Sass files.

Thanks!

[nex3]

Can you make a CL and export into PR (See http://go/mdc-web-g3sync)? You wouldn't have to do any additional changes on GitHub except exporting a CL. (Internally we've good test coverage including screenshot tests and global tests).

I tried running mdc import_pr 5491 but I got an error:

ERROR: Cannot find last imported revision. Use --force if you really want to proceed with the migration
  CAUSED BY: Cannot find reference '6fb7ccb68afea5ad782e79948764ca16f1b40468'

Can we also update actual _mixin, _variables files to use these index files, using index files greatly improves readability of our Sass files.

My main goal is to unblock downstream migrations as soon as possible. I'll try to make time to circle back to this later on, but it may need to wait until @jathak is back in the office and the team has more bandwidth again.

[abhiomkar]

I tried running mdc import_pr 5491 but I got an error:

ERROR: Cannot find last imported revision. Use --force if you really want to proceed with the migration
  CAUSED BY: Cannot find reference '6fb7ccb68afea5ad782e79948764ca16f1b40468'

Please use --force option to force snapshot the PR.

My main goal is to unblock downstream migrations as soon as possible. I'll try to make time to circle back to this later on, but it may need to wait until @jathak is back in the office and the team has more bandwidth again.

SGTM. Thanks!

[jathak]

• Can we also update actual _mixin, _variables files to use these index files, using index files greatly improves readability of our Sass files.

I don't think there's currently a simple way to do this automatically, but writing one shouldn't be too challenging (since it's essentially just merging the @use rules and changing namespaces), but since it's a change that's local to the file, it makes sense to do that separately, once the main index change is done.

[nex3]

Obsoleted by #5539