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.

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",
 ]