wic: align layer plugin path

wic look for external layer source plugin under scripts/lib/
while other tools look for lib/.

allow wic to check for source plugin at both scripts/lib/ and
lib/ to align with other tools while avoid breaking any existing
source plugin in external layer.

[YOCTO #13056]

Signed-off-by: Lee Chee Yang <chee.yang.lee@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>