plots: fix regressions to top-level plots

[dberenbaum]

Fixes regressions in #8421.

1. Makes plots work again when y is a list.

plots:
  plot1.csv:
    y:
      - acc
      - loss
    x: epoch

2. Supports y lists as one of multiple data sources like:

plots:
  plot:
    y:
      plot1.csv:
        - acc
        - loss
      plot2.csv:
        - loss
    x:
      epoch

@amritghimire @ssachkovskaya I tested with https://studio.iterative.ai/user/dberenbaum/projects/dvc_plots_tests-cvls4xeliq and 1 looks broken (2 seems okay). Not sure if it will get automatically fixed with this change or if something is needed on your end.

[codecov]

Codecov Report

Base: 93.52% // Head: 93.53% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (b7ebd6c) compared to base (23e4d37).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head b7ebd6c differs from pull request most recent head c4907fd. Consider uploading reports for the commit c4907fd to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #8703    +/-   ##
========================================
  Coverage   93.52%   93.53%            
========================================
  Files         457      457            
  Lines       36128    36265   +137     
  Branches     5222     5239    +17     
========================================
+ Hits        33790    33920   +130     
- Misses       1837     1844     +7     
  Partials      501      501            

Impacted Files	Coverage Δ
tests/unit/render/test_vega_converter.py	100.00% <ø> (ø)
dvc/render/converter/vega.py	91.20% <100.00%> (ø)
dvc/repo/index.py	91.95% <0.00%> (-1.09%)	⬇️
tests/func/test_repo_index.py	96.83% <0.00%> (+1.69%)	⬆️

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[amritghimire]

Tested with this version in studio. The plot works without any change in studio. Let me know if the following is not the desired response.

[dberenbaum]

Looks good @amritghimire!