by tstoeckler: Improve the API of library (registry -> definition discovery)

libraries.api.php

libraries.module

libraries.services.yml

...ternalLibrary/Definition/DefinitionDiscoveryInterface.php

src/ExternalLibrary/Definition/StreamDefinitionDiscovery.php

...ternalLibrary/Exception/LibraryClassNotFoundException.php

src/ExternalLibrary/LibraryManager.php

src/ExternalLibrary/LibraryManagerInterface.php

src/ExternalLibrary/LibraryType/LibraryTypeFactory.php

...ternalLibrary/LibraryType/LibraryTypeManagerInterface.php

src/ExternalLibrary/Registry/LibraryRegistryInterface.php

src/Plugin/libraries/LibraryType/PhpFileLibraryType.php

src/StreamWrapper/LibraryDefinitionsStream.php

tests/src/Kernel/ExternalLibrary/Asset/AssetLibraryTest.php

...src/Kernel/ExternalLibrary/PhpFile/PhpFileLibraryTest.php

tests/src/Kernel/LibraryKernelTestBase.php

tests/src/Kernel/LibraryTypeKernelTestBase.php