Discover should include a myke config only once

[pkuczynski]

I have a dir0/myke.yml config which references few others:

discover:
  - ../dir1
  - ../dir2
  - ../dir3

However dir1 also discovers dir2:

discover:
  - ../dir2

In such scenario, I will see duplicated dir2 entries when I run myke in dir0.

I would expect that discovery skips configs which have been already discovered...