deptrac: Enhancement: Ignored/Exempt Layers

I would like a way to define a layer that was available for dependency checks to other layers but otherwise exempt from validation. See also the conversation at: https://github.com/qossmic/deptrac/issues/506

This might be beyond me for my first PR but I would be willing to try. I imagine it working with a special modifier like ~ (maybe + as a nod to transitive dependencies?). Those this would be a valid depfile:

layers:
  - name: Core
    collectors:
      - type: className
        regex: Core
  - name: Module
    collectors:
      - type: className
        regex: Module

ruleset:
  Core: +
  Module:
    - Core