Enable using PSR-4 autoloader for MediaWiki core and extensions

Enable using PSR-4 autoloader for MediaWiki core and extensions

This adds support for a PSR-4 (<http://www.php-fig.org/psr/psr-4/>)
autoloader, so instead of needing to manually list each class, just the
namespace prefix is needed.

Extensions can set a "AutoloadNamespaces" property in extension.json to
register PSR-4 compatible namespaces to be autoloaded.

The implementation is based off of the example implementation
(<http://www.php-fig.org/psr/psr-4/examples/>) with some modifications
for performance, notably cutting down on function calls, and only trying
to look up classes that are namespaced.

The generateLocalAutoload.php script will ignore any directory that is
registered as a PSR-4 namespace.

Bug: T99865
Bug: T173799
Change-Id: Id095dde37cbb40aa424fb628bd3c94e684ca2f65