Revert "Remove AssetLibraryRepository."

This reverts commit f8330ac2f3d4e28e720335900c29438c4d563cc3.

Turns out we do need to bring it back.

Conflicts:
	core/lib/Drupal/Core/Asset/Collection/AssetCollection.php
	core/lib/Drupal/Core/Asset/Collection/AssetCollectionInterface.php
	core/tests/Drupal/Tests/Core/Asset/AssetCollectionTest.php
Unit tests for AssetLibraryRepository.