3. Rename Sass mixins in src/material/legacy-<component>

Perform the following steps to complete the rename and fix all references:
1. Global find & replace `@include mat.<component>` with `@include mat.<legacy-component>`
2. In `src/material/legacy-<component>/_<component>-legacy-index.scss` and `src/material/legacy-<component>/_<component>-theme.import.scss`, find & replace `mat-<component>` with `mat-legacy-<component>`
3. In `src/_index.scss`, find the line that says `@forward './legacy-<component>/<component>-theme' as`. Change every instance of `<component>` after the word `as` to `<legacy-component>`
4. Run `yarn format`

Author: mmalerba

src/material/_index.scss

@@ -69,7 +69,7 @@
 @forward './button/button-theme' as button-* show button-theme, button-color, button-typography;
 @forward './button-toggle/button-toggle-theme' as button-toggle-* show button-toggle-theme,
   button-toggle-color, button-toggle-typography;
-@forward './legacy-card/card-theme' as card-* show card-theme, card-color, card-typography;
+@forward './legacy-card/card-theme' as legacy-card-* show legacy-card-theme, legacy-card-color, legacy-card-typography;
 @forward './checkbox/checkbox-theme' as checkbox-* show checkbox-theme, checkbox-color,
   checkbox-typography;
 @forward './chips/chips-theme' as chips-* show chips-theme, chips-color, chips-typography;

src/material/legacy-card/_card-legacy-index.scss

@@ -1,2 +1,2 @@
 @forward 'card-theme' hide color, theme, typography;
-@forward 'card-theme' as mat-card-* hide mat-card-density;
+@forward 'card-theme' as mat-legacy-card-* hide mat-legacy-card-density;

src/material/legacy-card/_card-theme.import.scss

@@ -2,7 +2,7 @@
 @forward '../core/style/private.import';
 @forward '../core/typography/typography-utils.import';
 @forward 'card-theme' hide color, theme, typography;
-@forward 'card-theme' as mat-card-* hide mat-card-density;
+@forward 'card-theme' as mat-legacy-card-* hide mat-legacy-card-density;
 
 @import '../core/theming/palette';
 @import '../core/theming/theming';