Issue #2090623 by tstoeckler: Introduce the concept of locators

libraries.services.yml

src/Annotation/Locator.php

...xternalLibrary/Exception/LibraryNotInstalledException.php

src/ExternalLibrary/ExternalLibraryManager.php

src/ExternalLibrary/Local/LocalLibraryInterface.php

src/ExternalLibrary/Local/LocalLibraryTrait.php

src/ExternalLibrary/Local/LocatorInterface.php

src/ExternalLibrary/Local/LocatorManager.php

src/ExternalLibrary/LocalLibraryInterface.php

src/ExternalLibrary/PhpFile/PhpFileLibrary.php

src/ExternalLibrary/PhpFile/PhpFileLibraryInterface.php

src/Plugin/MissingPluginConfigurationException.php

src/Plugin/libraries/Locator/StreamLocator.php