plots: support x-dict in nested dvc.yaml

[mattseddon]

• [x] ❗ I have followed the Contributing to DVC checklist.

• [x] 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

---

This PR fixes a bug in the collection of plots definitions. The bug exists when there is a plot collected from a nested dvc.yaml that uses separate files for the x and y values.

e.g.

- Error vs max_leaf_nodes:
    template: simple
    x: 
      dvclive/plots/metrics/Max_Leaf_Nodes.tsv: Max_Leaf_Nodes
    y:
      dvclive/plots/metrics/Error.tsv: Error

in {root}/pipelines/data-increment/dvc.yaml

Previously the path to the x file was not updated to take into account the path to the dvc.yaml that it exists in.

For more context see this thread.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.69%. Comparing base (84a1750) to head (93403de).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #10318 +/- ## ========================================== + Coverage 90.67% 90.69% +0.02% ========================================== Files 499 499 Lines 38459 38504 +45 Branches 5552 5555 +3 ========================================== + Hits 34871 34921 +50 + Misses 2949 2946 -3 + Partials 639 637 -2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.