Restructure the branch-rules.yml to be keyed by projects instead of by branch layers.

We want to move branch info to one place, see sdk/kde-builder#34 (moved).
Would it be ok to reformat branch-rules.yml like in the logical-module-structure.json? Something like this:

frameworks/*: 
  kf6: "master"
  kf5: "kf5"
  stable-kf5: "kf5"
  stable-kf6: "master"
frameworks/kcolorscheme: 
  kf6: "master"
  kf5: ""
  stable-kf5: ""
  stable-kf6: "master"
frameworks/kstatusnotifieritem: 
  kf6: "master"
  kf5: ""
  stable-kf5: ""
  stable-kf6: "master"

In my opinion, this would improve readability.