* 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
* remote: move upload()/download() into tree
* remote: move path_info/path_cls into tree
* remote.azure: finish moving methods into tree
* remote.gs: finish moving methods into tree
* remote.hdfs: finish moving methods to tree
* remote.http: finish moving methods into tree
* remote.oss: finish moving methods into tree
* remote.s3: finish moving methods into tree
* remote.ssh: finish moving methods into tree
* remote.gdrive: finish moving methods into tree
* tests: update remote unit tests
* tests: update func tests
* remote: use walk_files() for all remotes
* list_cache_paths() now uses tree.walk_files() for all remotes except
local/ssh
* fix DS warnings
* bugfixes
* BaseRemote: start moving tree functions into BaseRemoteTree
* remote: add LocalRemoteTree
* remote: add cloud remote trees
* remote: move remove(), makedirs() into tree
* remote: move move(), copy(), symlink(), hardlink(), reflink() into tree
* fix attributes for moved remote methods
* tests: update unit tests for moved remote tree methods
* tests: mv unit/remote/test_remote_dir.py unit/remote/test_remote_tree.py
* fix attributes for moved remote tree functions
* tests: update func tests for moved remote tree methods
* test fixes
* fix DS warnings
Batuhan Taskaya
Ruslan Kuprieiev
Saugat Pachhai
Peter Rowlands (변기호)