plots: support x-dict in nested dvc.yaml

[mattseddon]

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• 📖 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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     

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