Override USWDS Sass packages at source

[aduth]

This builds upon #336 to further restructure the folder structure to align to how files are managed in the upstream project.

Why?

• Improved performance. Improves build time for the default package by nearly 50% (8.8s to 4.7s)
  • See related comment at USWDS - USWDS: Forward dependencies by entrypoint reference uswds/uswds#5241 (comment)
• Better maintainability. Avoids needing to keep pacakges/_uswds.scss in sync as the upstream USWDS adds new components over time.
• Coverage for different import semantics. Specifically, supports both import of the named package, as well as the "Using Package Source" performance optimization.
• Clearer separation. Overrides are strictly separate from the base styles.

Unfortunately the diff didn't turn out as clean as I'd hoped, despite the fact that 99% of changes are file renames.

Essentially, this converts:

Before:

src/
 - scss/
  - packages/
   - _usa-accordion.scss

// _usa-accordion.scss

@forward './usa-accordion/index';

.usa-accordion {
  // ...customizations...
}

After:

src/
 - scss/
  - packages/
   - _usa-accordion/
    - src/
     -  _styles.scss
     - _overrides.scss
   - _usa-accordion.scss

// _usa-accordion.scss

@forward 'usa-accordion/index';
@forward 'usa-accordion/src/overrides';

// usa-accordion/src/_styles.scss

@forward 'usa-accordion/src/styles/index';
@forward './overrides';

// usa-accordion/src/_overrides.scss

.usa-accordion {
  // ...customizations...
}

The same general principle of intercepting USWDS files applies, but now applies for both the top-level import (@forward 'usa-accordion') as well as the package source (@forward 'usa-accordion/src/styles').

The only meaningful code change here was to eliminate $border-width variable in some of the SCSS files. I was running into an error message that seemed to be caused by multiple forwarded modules each defining the variable. Unclear why this wasn't an issue previously 😬

Error: Two forwarded modules both define a variable named $border-width.
  ╷
1 │ @forward 'usa-checkbox/index';
  │ ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ original @forward
2 │ @forward 'usa-checkbox/src/overrides';
  │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ new @forward
  ╵
  _usa-checkbox.scss 2:1               @forward
  uswds-form-controls/_index.scss 4:1  @forward
  _uswds-form-controls.scss 2:1        @forward
  uswds/_index.scss 51:1               @forward
  _uswds.scss 2:1                      @forward
  src/scss/styles.scss 2:1             root stylesheet

[aduth]

I'm open to trying to break this down into a few separate pull requests to try to coerce git into showing a simpler diff, if it'd be helpful for review.

[zachmargolis]

LGTM

it might have been nicer as one PR per component or maybe even separate commits (one commit per compoonent) but the documentatino makes it clear.