refactor: consolidate tree classes #4050
Description
Follow up to #3882.
Currently we have tree classes that all inherit from dvc.scm.tree.BaseTree spread out all over the code base w/DvcTree, RepoTree, SCM/git related trees, BaseRemoteTree, and all of the associated cloud remote trees. It may make more sense to group these in something like dvc.tree and dvc.tree.cloud. This would also allow us to simplify what is currently under dvc.remote, since we really only need single files for Remote and CloudCache now. (Cloud cache code could also potentially just be moved into dvc.cache now as well.)