externalsrc.bbclass was moved to classes-recipe as part of oe-core f5c128008365e141082c129417eb72d2751e8045, but it can be used in both global and recipe scope. The class should be moved back to classes/.
https://lore.kernel.org/openembedded-core/20221019203751.2265757-1-ross.burton@arm.com/T/#u
Merged in 7a2edcd4b7cb5a2d829289a11eff62663268fbf3.