oeqa/runtime: load modules using importlib

Instead of using __import__() which is low-level and discouraged, use
importlib.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>