Fix extension registration support

Fix global variable naming issue.

Also match changes done to SemanticForms.php in extensions.json

Also do some other changes to get this to work properly.

Also remove autoload from composer.json ( See
https://git.wikimedia.org/commitdiff/mediawiki%2Fskins%2FVector.git/28c2e986ae5e5ac0fd43bde3d7a057b7c8771b77 )

Also added a note to both files to explain doint forget to update both
files with the same changes.

Bug: T121322
Change-Id: I5dd6eae5a1ae0b4e094ad3d0b254dc320cc032d6