Consolidate trees (#4243)

* 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

dvc/dependency/https.py

@@ -1,6 +1,6 @@
-from ..tree.https import HTTPSRemoteTree
+from ..tree.https import HTTPSTree
 from .http import HTTPDependency
 
 
 class HTTPSDependency(HTTPDependency):
-    TREE_CLS = HTTPSRemoteTree
+    TREE_CLS = HTTPSTree