fix(material/core): ensure all forward statements are relative (#33627)

Fixes a couple of `@forward` statements that didn't start with `./`.

Fixes #32917.

Author: crisbeto

src/material/_index.scss

@@ -18,10 +18,10 @@
 @forward './core/typography/typography' show typography-hierarchy;
 @forward './core/typography/typography-utils' show font-shorthand;
 @forward './core/theming/typography' as typography-*;
-@forward 'core/tokens/system' show system-level-colors,
+@forward './core/tokens/system' show system-level-colors,
     system-level-typography, system-level-elevation, system-level-shape,
     system-level-motion, system-level-state, theme, theme-overrides, m2-theme;
-@forward 'core/tokens/classes' show system-classes;
+@forward './core/tokens/classes' show system-classes;
 
 // Private/Internal
 @forward './core/density/private/all-density' show all-component-densities;