refactor: migrate to the Sass module system

[jathak]

This PR was generated by running this script based on the Sass migrator. It migrates all components to the Sass module system, removing manual mdc-$component prefixes and implicit dependencies between files (though still leaving both intact for users still using @import).

High-level overview of the changes made here:

• Prefixes (e.g. mdc-button-) for each component have been removed from all variables, functions, and mixins within the code. Prefixes on CSS selectors are unaffected.
• When depending on one of a component's files via @use, only members directly declared in that file are available. The mdc-$component.scss files should only expose CSS, not Sass members.
• Import-only files have been generated that add back all of the prefixes and explicitly forward all members that were implicitly available via @import previously, so existing users that still use @import shouldn't break as a result of these changes. These files will then be used by the migrator when those users later migrate to the module system themselves.
• Minor breaking change: Four variables and a mixin in mdc-textfield were renamed to use a mdc-text-field- prefix when depended on via @import. None of the variables are used within Google and the mixin (formerly mdc-required-text-field-label-asterisk_, now required-label-asterisk_) is used only once by MWC.

[jathak]

One other note about the script used here: while the underlying Sass migrator is fairly robust, the script itself is fragile, as preserving the proper prefixes for some of the more complex components (e.g. mdc-ripple, mdc-select, mdc-textfield) required a bunch of sed transformations, some of which reference specific lines to change. We can reuse the script if there are enough changes to master before this is submitted to make a merge difficult, but future runs may require tweaks to it.

[codecov-io]

Codecov Report

Merging #5453 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5453      +/-   ##
==========================================
- Coverage   97.06%   97.04%   -0.02%     
==========================================
  Files         164      164              
  Lines        6266     6266              
  Branches      824      782      -42     
==========================================
- Hits         6082     6081       -1     
- Misses        184      185       +1

Impacted Files	Coverage Δ
testing/dom/events.ts	71.42% <0%> (-28.58%)	⬇️
packages/mdc-auto-init/index.ts	96% <0%> (+4%)	⬆️

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[jathak]

Okay, the lint issue was that stylelint-scss was out of date. The unit tests seem to be failing because of an issue with Chrome

[jathak]

cl/289132951

[abhiomkar]

🎉