import/import-url: try not to delete dvcfile before import/import-url completes

[skshetry]

• ❗ 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. 🙏

Related to #3362.

[codecov]

Codecov Report

Base: 93.64% // Head: 93.72% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (51cdc6c) compared to base (61afdca).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8876      +/-   ##
==========================================
+ Coverage   93.64%   93.72%   +0.07%     
==========================================
  Files         453      453              
  Lines       36101    36141      +40     
  Branches     5237     5239       +2     
==========================================
+ Hits        33808    33872      +64     
+ Misses       1787     1761      -26     
- Partials      506      508       +2     

Impacted Files	Coverage Δ
dvc/repo/imp_url.py	80.95% <100.00%> (-2.03%)	⬇️
tests/unit/output/test_output.py	100.00% <0.00%> (ø)
tests/unit/stage/test_serialize_pipeline_lock.py	100.00% <0.00%> (ø)
tests/func/test_add.py	98.72% <0.00%> (+0.01%)	⬆️
dvc/stage/__init__.py	94.70% <0.00%> (+0.63%)	⬆️
dvc/stage/serialize.py	94.91% <0.00%> (+2.60%)	⬆️
dvc/output.py	90.26% <0.00%> (+2.84%)	⬆️

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