build: add lint rule for unused Sass imports (#22454)

Since all the imports are namespaced now, it's trivial to figure out whether they're being used. These changes add a custom lint rule and fix all of the failures.

Author: crisbeto

.stylelintrc.json

@@ -10,7 +10,8 @@
     "./tools/stylelint/no-top-level-ampersand-in-mixin.ts",
     "./tools/stylelint/theme-mixin-api.ts",
     "./tools/stylelint/no-import.ts",
-    "./tools/stylelint/single-line-comment-only.ts"
+    "./tools/stylelint/single-line-comment-only.ts",
+    "./tools/stylelint/no-unused-import.ts"
   ],
   "rules": {
     "material/no-prefixes": [true, {
@@ -20,6 +21,7 @@
     "material/theme-mixin-api": true,
     "material/selector-no-deep": true,
     "material/no-nested-mixin": true,
+    "material/no-unused-import": true,
     "material/single-line-comment-only": [true, {
       "filePattern": "\\.scss$"
     }],

src/material-experimental/column-resize/_column-resize-theme.scss

@@ -1,7 +1,6 @@
 @use 'sass:map';
 @use '../../material/core/style/variables';
 @use '../../material/core/style/vendor-prefixes';
-@use '../../material/core/theming/palette';
 @use '../../material/core/theming/theming';
 
 @mixin color($config-or-theme) {

src/material-experimental/mdc-autocomplete/autocomplete.scss

@@ -1,7 +1,6 @@
 @use '@material/menu-surface/mixins' as mdc-menu-surface;
 @use '@material/list/mixins' as mdc-list;
 @use '../../cdk/a11y';
-@use '../mdc-helpers/mdc-helpers';
 
 @include mdc-menu-surface.core-styles($query: structure);
 

src/material-experimental/mdc-checkbox/checkbox.scss

@@ -1,5 +1,4 @@
 @use '@material/checkbox' as mdc-checkbox;
-@use '@material/checkbox/checkbox-theme' as mdc-checkbox-theme;
 @use '@material/form-field' as mdc-form-field;
 @use '@material/ripple' as mdc-ripple;
 @use 'sass:map';

src/material-experimental/mdc-form-field/_form-field-native-select.scss

@@ -1,6 +1,5 @@
 @use 'sass:map';
 @use '../mdc-helpers/mdc-helpers';
-@use '../../material/core/theming/theming';
 @use '../../cdk/a11y';
 @use '../../material/core/theming/palette';
 @use '@material/theme/theme-color' as mdc-theme-color;

src/material-experimental/mdc-form-field/_form-field-theme.scss

@@ -1,4 +1,3 @@
-@use '@material/ripple/mixins' as mdc-ripple;
 @use '@material/textfield' as mdc-textfield;
 @use '@material/floating-label' as mdc-floating-label;
 @use '@material/notched-outline' as mdc-notched-outline;

src/material-experimental/mdc-form-field/_mdc-text-field-textarea-overrides.scss

@@ -1,5 +1,3 @@
-@use 'form-field-sizing';
-
 // MDCs default textarea styles cannot be used because they only apply if a special
 // class is applied to the "mdc-text-field" wrapper. Since we cannot know whether the
 // registered form-field control is a textarea and MDC by default does not have styles

src/material-experimental/mdc-helpers/_focus-indicators.scss

@@ -1,6 +1,5 @@
 @use 'sass:map';
 @use '../../material/core/style/layout-common';
-@use '../../material/core/focus-indicators/focus-indicators';
 
 /// Mixin that turns on strong focus indicators.
 ///

src/material-experimental/mdc-helpers/_mdc-helpers.scss

@@ -4,10 +4,8 @@
 
 @use '@material/feature-targeting' as mdc-feature-targeting;
 @use '@material/typography' as mdc-typography;
-@use '@material/theme/functions' as mdc-theme-functions;
 @use '@material/theme/theme-color' as mdc-theme-color;
 @use 'sass:map';
-@use '../../material/core/style/layout-common';
 @use '../../material/core/theming/theming';
 @use '../../material/core/typography/typography';
 @use '../../material/core/typography/typography-utils';

src/material-experimental/mdc-list/_interactive-list-theme.scss

@@ -1,6 +1,5 @@
 @use '@material/ripple' as mdc-ripple;
 @use 'sass:map';
-@use '../mdc-helpers/mdc-helpers';
 @use '../../material/core/theming/theming';
 
 // Mixin that provides colors for the various states of an interactive list-item. MDC