dvc: use flatten-dict instead of flatten-json (#4500)

* dvc: use flatten-dict instead of flatten-json

Fixes #4499

* utils: add [un]flatten

* pyinstaller: add flatten-dict hook

setup.py

@@ -75,7 +75,7 @@ install_requires = [
     "pydot>=1.2.4",
     "speedcopy>=2.0.1; python_version < '3.8' and sys_platform == 'win32'",
     "dataclasses; python_version < '3.7'",
-    "flatten_json>=0.1.6,<0.1.8",
+    "flatten_dict>=0.3.0,<1",
     "tabulate>=0.8.7",
     "pygtrie==2.3.2",
     "dpath>=2.0.1,<3",