params: use dpath for xpaths (#3531)

Currently `dvc run -p angles` is broken when you use it with `params.yaml`
that looks like this:

```
angles:
    - 2
    - 3
    - 100
    - 3.2
    - 36.2
    - 3.6
```

because we are incorrectly using `flatten` instead of simply using some
xpath-like library to access params that are specified.

dvc/dependency/param.py

@@ -2,9 +2,8 @@ import os
 import yaml
 from collections import defaultdict
 
+import dpath.util
 from voluptuous import Any
-from funcy import select_keys
-from flatten_json import flatten
 
 from dvc.compat import fspath_py35
 from dvc.dependency.local import DependencyLOCAL
@@ -87,9 +86,10 @@ class DependencyPARAMS(DependencyLOCAL):
                     "Unable to read parameters from '{}'".format(self)
                 ) from exc
 
-        config = flatten(config, ".")
-
-        return select_keys(lambda key: key in self.params, config)
+        ret = {}
+        for param in self.params:
+            ret[param] = dpath.util.get(config, param, separator=".")
+        return ret
 
     def save_info(self):
         info = self._get_info()

setup.py

@@ -81,6 +81,7 @@ install_requires = [
     "flatten_json>=0.1.6",
     "texttable>=0.5.2",
     "pygtrie==2.3.2",
+    "dpath>=2.0.1,<3",
 ]
 
 

tests/unit/dependency/test_params.py

@@ -70,9 +70,11 @@ def test_get_info_unsupported_format(tmp_dir, dvc):
 
 
 def test_get_info_nested(tmp_dir, dvc):
-    tmp_dir.gen("params.yaml", yaml.dump({"some": {"path": {"foo": "val"}}}))
+    tmp_dir.gen(
+        "params.yaml", yaml.dump({"some": {"path": {"foo": ["val1", "val2"]}}})
+    )
     dep = DependencyPARAMS(Stage(dvc), None, ["some.path.foo"])
-    assert dep._get_info() == {"some.path.foo": "val"}
+    assert dep._get_info() == {"some.path.foo": ["val1", "val2"]}
 
 
 def test_save_info_missing_params(dvc):