import: fix broken progressbar when importing folder

[petrchmelar]

Add child progress bar to dvc fs to report directory import progress correctly.

Fixes #10677

• ❗ I have followed the Contributing to DVC checklist.

• 📖 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. 🙏

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.07%. Comparing base (2431ec6) to head (e8a680d).
Report is 27 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10678      +/-   ##
==========================================
+ Coverage   90.68%   91.07%   +0.38%     
==========================================
  Files         504      504              
  Lines       39795    39930     +135     
  Branches     3141     3157      +16     
==========================================
+ Hits        36087    36365     +278     
+ Misses       3042     2938     -104     
+ Partials      666      627      -39     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[skshetry]

Thank you, @petrchmelar for the contributions. I made one minor adjustment to your PR on how callback.relative_update() gets called.

I also found a small bug that was fixed in iterative/dvc-data#596.
Benchmark showed a small change but nothing major for get/import.

Single vs multiple progress bars can be tackled separately.