exp init: nested parameters

[dberenbaum]

Bug Report

Description

dvc exp init does not read nested parameters.

Reproduce

$ echo "foo: \n  bar: 1" > params.yaml
$ dvc exp init python train.py
Created default stage in dvc.yaml.

To run default stage, use "dvc exp run".
$ cat dvc.yaml
stages:
  default:
    cmd: python train.py
    deps:
    - data
    - src
    params:
    - foo
    outs:
    - models
    metrics:
    - metrics.json:
        cache: false
    plots:
    - plots:
        cache: false

Expected

$ cat dvc.yaml
stages:
  default:
    cmd: python train.py
    deps:
    - data
    - src
    params:
    - foo.bar
    outs:
    - models
    metrics:
    - metrics.json:
        cache: false
    plots:
    - plots:
        cache: false

Environment information

Output of dvc doctor:

$ dvc doctor
DVC version: 2.7.4+38d758.mod
---------------------------------
Platform: Python 3.9.5 on macOS-11.5.2-arm64-arm-64bit
Supports:
        http (aiohttp = 3.7.4.post0, aiohttp-retry = 2.4.5),
        https (aiohttp = 3.7.4.post0, aiohttp-retry = 2.4.5),
        s3 (s3fs = 2021.8.0, boto3 = 1.17.5)
Cache types: <https://error.dvc.org/no-dvc-cache>
Caches: local
Remotes: local
Workspace directory: apfs on /dev/disk3s1s1
Repo: dvc, git

[skshetry]

This feels like correct behaviour to me. The child values of foo will be still get tracked.

Instead of depending on leaf values, it uses root values.
Whenever foo is changed (if a new child value is added for example), it'll get tracked.

This is closer to #6605.