KDevelop: keep using extragear/ prefix in legacy projectpath
Some build setups still rely on that, also was the change to kde/ incomplete.
Given the projectpath is to be dropped soonish, let's just keep it for now.
At least IIRC only jenkins CI setup and perhaps some parts of kdesrc-build are the last ones relying on it? And it is not needed for the setup of release service/KDE Gear?