fix(icon)!: rename "lib" directory to "internal"

BREAKING CHANGE: Rename `@material/web/icon/lib` to `@material/web/icon/internal`. Prefer not using internal files.

PiperOrigin-RevId: 550579714

icon/_icon.scss

@@ -3,4 +3,4 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
-@forward './lib/icon' show theme;
+@forward './internal/icon' show theme;

icon/icon.ts

@@ -6,8 +6,8 @@
 
 import {customElement} from 'lit/decorators.js';
 
-import {Icon} from './lib/icon.js';
-import {styles} from './lib/icon-styles.css.js';
+import {Icon} from './internal/icon.js';
+import {styles} from './internal/icon-styles.css.js';
 
 declare global {
   interface HTMLElementTagNameMap {