fix: key the import file cache on the source repo revision

[adarshsm]

Fixes #11111

Bug

For a --no-download repo import, dvc pull stores the fetched files under .dvc/cache/fs/<protocol>/<token>. The token is built in _load_storage_from_import (dvc/repo/index.py) as tokenize(dep.fs_path, meta_token). A RepoDependency records no meta, so meta_token is the same constant for every revision and every source repo. That causes two problems:

1. Stale data after update (the reported case). import --no-download --rev v1 → pull → update --no-download --rev main → pull checks out the v1 data. .dvc/cache/fs already has an entry under the same token, so the new rev_lock is never fetched.
2. Data from the wrong repo. dvc import --no-download ../srcA data -o a plus dvc import --no-download ../srcB data -o b, then pulling both, gives b/ the contents of srcA. Both imports land on the same token.

I reproduced both with the CLI on current main. I only saw this with directory imports. A single-file import with the same steps already picked up the new content.

Fix

For repo imports, build the token from the source repo url and rev_lock instead of the empty dep meta. Every source revision now gets its own cache entry. Other imports (import-url, version-aware) are unchanged.

Entries written under the old token are just never looked up again. Nothing reads them, but they aren't cleaned up either.

Tests

Two new tests in tests/func/test_import.py:

• test_pull_import_no_download_after_update: covers the scenario from the issue.
• test_pull_import_no_download_same_path_from_two_repos: covers the cross-repo collision.

Both fail on main and pass with this change. tests/func/test_import.py, test_update.py, test_repo_index.py, test_data_cloud.py, test_import_url.py, test_external_repo.py, test_checkout.py, test_status.py and tests/func/api pass locally: 278 passed. ruff check and ruff format --check are clean.

• ❗ 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. (No docs change needed.)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.98%. Comparing base (2431ec6) to head (2071ebb).
⚠️ Report is 213 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11112      +/-   ##
==========================================
+ Coverage   90.68%   90.98%   +0.30%     
==========================================
  Files         504      505       +1     
  Lines       39795    41163    +1368     
  Branches     3141     3264     +123     
==========================================
+ Hits        36087    37452    +1365     
- Misses       3042     3072      +30     
+ Partials      666      639      -27     

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