scap backport: major refactor

Refactor of scap backport to improve readability/editability

• Creates classes for a single change and a collection of changes
• Creates a class to manage git repos and submodules cache
• Updates handling of multiple dependencies with same change-id:
  • Allows backport to continue if all dependencies with the same change-id have been merged
  • Selects a dependency from list if branches are the same or one has already been selected for backport

Bug: T334984