* dvc: get rid of is_walking_tree
No longer needed. Some applications of `isinstance()` will be refactored
in the future, as trees will just provide the needed functionality.
* dvc: move GitTree to dvc/tree/
* dvc: rename *RemoteTree to *Tree
* remote: move constants into trees
* remote: replace Remote() Cache() helper functions
* output: update for remote changes
* cache/data_cloud: update for remote changes
* dependency: update for remote changes
* stage: update for remote changes
* remote: move remaining non-index methods into tree
* tests: update remote unit tests
* remote: LocalCache needs to lock remote index, not cache index
* fix bugs from moved PARAM_CHECKSUM/PARAM_RELPATH
* tests: repo tree open needs state context
* remote: make Remote.gc a class method
* operates on obj.tree, so that GC can be done with both remote and
cache instances
* bug fixes
* tests: update test_data_cloud
* bug fixes
* tests: update func tests