Improve action lookup

The code here is extremely twisty, but at the end of the day it
ends up creating the path to an action's input -- which we can
readily get from schema. Simplify the logic, speeding it up
significantly.

Change-Id: Ic29689bd03b99b5a2095efa4e3fb3866afc08526
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit c566755c9040b150261b989800331b12d52137da)