fix: remove imports from top-level package (#604)

Removes the imports from `@angular/material` since they end up breaking some of the examples once they're forked to Stackblitz.

Author: crisbeto

material.angular.io/src/assets/stackblitz/main.ts

@@ -3,7 +3,7 @@ import './polyfills';
 import {HttpClientModule} from '@angular/common/http';
 import {NgModule} from '@angular/core';
 import {FormsModule, ReactiveFormsModule} from '@angular/forms';
-import {MatNativeDateModule} from '@angular/material';
+import {MatNativeDateModule} from '@angular/material/core';
 import {BrowserModule} from '@angular/platform-browser';
 import {platformBrowserDynamic} from '@angular/platform-browser-dynamic';
 import {BrowserAnimationsModule} from '@angular/platform-browser/animations';

material.angular.io/src/assets/stackblitz/material-module.ts

@@ -1,48 +1,45 @@
+import {NgModule} from '@angular/core';
 import {A11yModule} from '@angular/cdk/a11y';
 import {DragDropModule} from '@angular/cdk/drag-drop';
 import {PortalModule} from '@angular/cdk/portal';
 import {ScrollingModule} from '@angular/cdk/scrolling';
 import {CdkStepperModule} from '@angular/cdk/stepper';
 import {CdkTableModule} from '@angular/cdk/table';
 import {CdkTreeModule} from '@angular/cdk/tree';
-import {NgModule} from '@angular/core';
-import {
-  MatAutocompleteModule,
-  MatBadgeModule,
-  MatBottomSheetModule,
-  MatButtonModule,
-  MatButtonToggleModule,
-  MatCardModule,
-  MatCheckboxModule,
-  MatChipsModule,
-  MatDatepickerModule,
-  MatDialogModule,
-  MatDividerModule,
-  MatExpansionModule,
-  MatGridListModule,
-  MatIconModule,
-  MatInputModule,
-  MatListModule,
-  MatMenuModule,
-  MatNativeDateModule,
-  MatPaginatorModule,
-  MatProgressBarModule,
-  MatProgressSpinnerModule,
-  MatRadioModule,
-  MatRippleModule,
-  MatSelectModule,
-  MatSidenavModule,
-  MatSliderModule,
-  MatSlideToggleModule,
-  MatSnackBarModule,
-  MatSortModule,
-  MatStepperModule,
-  MatTableModule,
-  MatTabsModule,
-  MatToolbarModule,
-  MatTooltipModule,
-  MatTreeModule,
-} from '@angular/material';
+import {MatAutocompleteModule} from '@angular/material/autocomplete';
+import {MatBadgeModule} from '@angular/material/badge';
+import {MatBottomSheetModule} from '@angular/material/bottom-sheet';
+import {MatButtonModule} from '@angular/material/button';
+import {MatButtonToggleModule} from '@angular/material/button-toggle';
+import {MatCardModule} from '@angular/material/card';
+import {MatCheckboxModule} from '@angular/material/checkbox';
+import {MatChipsModule} from '@angular/material/chips';
+import {MatStepperModule} from '@angular/material/stepper';
+import {MatDatepickerModule} from '@angular/material/datepicker';
+import {MatDialogModule} from '@angular/material/dialog';
+import {MatDividerModule} from '@angular/material/divider';
+import {MatExpansionModule} from '@angular/material/expansion';
+import {MatGridListModule} from '@angular/material/grid-list';
+import {MatIconModule} from '@angular/material/icon';
+import {MatInputModule} from '@angular/material/input';
+import {MatListModule} from '@angular/material/list';
+import {MatMenuModule} from '@angular/material/menu';
+import {MatNativeDateModule, MatRippleModule} from '@angular/material/core';
+import {MatPaginatorModule} from '@angular/material/paginator';
+import {MatProgressBarModule} from '@angular/material/progress-bar';
+import {MatProgressSpinnerModule} from '@angular/material/progress-spinner';
+import {MatRadioModule} from '@angular/material/radio';
+import {MatSelectModule} from '@angular/material/select';
+import {MatSidenavModule} from '@angular/material/sidenav';
+import {MatSliderModule} from '@angular/material/slider';
+import {MatSlideToggleModule} from '@angular/material/slide-toggle';
+import {MatSnackBarModule} from '@angular/material/snack-bar';
+import {MatSortModule} from '@angular/material/sort';
+import {MatTableModule} from '@angular/material/table';
+import {MatTabsModule} from '@angular/material/tabs';
+import {MatToolbarModule} from '@angular/material/toolbar';
+import {MatTooltipModule} from '@angular/material/tooltip';
+import {MatTreeModule} from '@angular/material/tree';
 
 @NgModule({
   exports: [