refactor(material/legacy-button): rename ts symbols

Author: wagnermaciel

src/material/legacy-button/public-api.ts

@@ -6,5 +6,5 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-export * from './button-module';
-export * from './button';
+export {MatLegacyButtonModule} from './button-module';
+export {MatLegacyButton, MatLegacyAnchor} from './button';

src/material/legacy-button/testing/public-api.ts

@@ -6,5 +6,5 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-export * from './button-harness';
-export * from './button-harness-filters';
+export {MatLegacyButtonHarness} from './button-harness';
+export {LegacyButtonHarnessFilters} from './button-harness-filters';