Add step mocking for composite actions and reusable workflows in the same repository

[shubhbapna]

Feature request

Right now we can only mock the steps in the actual workflow. However, we should be able to mock any resuable workflow or composite action in the same repository which is being used by the workflow we want to test

[Tapiiri]

+1 to this! Using composite actions seems such a smart design direction, so it's a shame this library quite essentially doesn't end up encouraging it...