fetch: use index fetch

[efiop]

Part of #9333

Notable improvements:

• All files are parallelized now, no matter if they are regular files, cloud versioning or imports.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.14 ⚠️

Comparison is base (1cb371d) 90.67% compared to head (63c0a3c) 90.54%.

❗ Current head 63c0a3c differs from pull request most recent head a6701d7. Consider uploading reports for the commit a6701d7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9424      +/-   ##
==========================================
- Coverage   90.67%   90.54%   -0.14%     
==========================================
  Files         470      470              
  Lines       35944    35886      -58     
  Branches     5180     5171       -9     
==========================================
- Hits        32593    32493     -100     
- Misses       2755     2800      +45     
+ Partials      596      593       -3     

Impacted Files	Coverage Δ
dvc/repo/pull.py	100.00% <ø> (ø)
dvc/repo/worktree.py	9.75% <ø> (+0.03%)	⬆️
tests/func/test_import_url.py	100.00% <ø> (ø)
tests/func/test_virtual_directory.py	100.00% <ø> (ø)
dvc/repo/fetch.py	93.93% <100.00%> (+22.92%)	⬆️
dvc/repo/index.py	92.80% <100.00%> (+0.05%)	⬆️
tests/func/test_data_cloud.py	99.42% <100.00%> (ø)
tests/func/test_import.py	99.43% <100.00%> (-0.02%)	⬇️

... and 5 files with indirect coverage changes

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[skshetry]

Is this an intentional change or an oversight? Previously, we could fetch subset of files, but after this PR, we fetch everything.

I was not aware about this change, which I just noticed when someone asked in https://discuss.dvc.org/t/why-dvc-pulls-full-dataset-instead-of-a-single-file/1897.

This breaks an important scenario for virtual directory, see https://dvc.org/doc/user-guide/data-management/modifying-large-datasets#modifying-remote-datasets.

Do we have no means to fetch a subset, @efiop?