convertExtensionToRegistration: Detect if composer autoloader is needed

convertExtensionToRegistration: Detect if composer autoloader is needed

If an extension specifies a require section in its composer.json, which contains
some dependencies, we can assume that this extension needs to load the composer
autoloader.

The maintenance script convertExtensionToRegistration.php will now check if the
extension has such a composer.json and, if so, adds the load_composer_autoloader
property in extension.json, which will try to load the autoloader of composer
is it is present.

Also add a check for existence of a require section in ComposerJson library
(ComposerJson::getRequiredDependencies()).

Bug: T119766
Change-Id: Icdbc37abc44e642afee2aab4c0e9298d3471124d