by tstoeckler: Document library types and rework asset libraries

libraries.api.php

libraries.module

src/ExternalLibrary/Asset/AssetLibrary.php

src/ExternalLibrary/Asset/AssetLibraryInterface.php

...ary/Asset/AttachableAssetLibraryRegistrationInterface.php

src/ExternalLibrary/Asset/LocalRemoteAssetTrait.php

src/ExternalLibrary/Asset/MultipleAssetLibrary.php

src/ExternalLibrary/Asset/MultipleAssetLibraryInterface.php

src/ExternalLibrary/Asset/SingleAssetLibraryTrait.php

...ternalLibrary/Definition/DefinitionDiscoveryInterface.php

src/ExternalLibrary/Definition/StreamDefinitionDiscovery.php

src/ExternalLibrary/Dependency/DependentLibraryInterface.php

src/ExternalLibrary/LibraryBase.php

src/ExternalLibrary/LibraryInterface.php

src/ExternalLibrary/LibraryManager.php

src/ExternalLibrary/PhpFile/PhpFileLibrary.php

src/ExternalLibrary/PhpFile/PhpFileLibraryInterface.php

src/ExternalLibrary/Type/LibraryTypeBase.php

src/ExternalLibrary/Version/VersionDetectorInterface.php

src/ExternalLibrary/Version/VersionedLibraryInterface.php