Can't disable mergify auto-labeling

[armanbilge]

Setting ThisBuild / mergifyLabelPaths := Map.empty doesn't disable labeling.

[armanbilge]

The problem is that projects automatically add themselves to mergifyLabelPaths and setting it to empty doesn't prevent that. This is easy to fix by adding another private setting for projects to add themselves to, then setting mergifyLabelPaths to that by default. This way it can be overriden.