legacy: lazy-import legacy classes inside v1 branches only, refs #9556

[mr-raj12]

Removes top-level LegacyRepository, LegacyRemoteRepository, and RepoObj1 imports from three files, moving each to the branch that uses it, refs #9556.

• borg.remote: LegacyRepository imported inside RepositoryServer.open(), guarded by v1_or_v2
• borg.archiver._common: LegacyRemoteRepository and LegacyRepository imported inside get_repository() under the same guard; RepoObj1 imported inside with_other_repository() under repository.version == 1
• borg.archiver.transfer_cmd: LegacyRepository imported at the top of transfer_chunks(), where it appears in the except tuple

No logic changes. None of these modules pull in borg.legacy at import time anymore, so a future phase can drop the package without them breaking at startup.

Refs #9556

Checklist

• PR is against master
• New code has tests
• Tests pass
• Commit message references related issue

[codecov]

Codecov Report

❌ Patch coverage is 64.28571% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.05%. Comparing base (6659bd9) to head (9fa3b5f).
⚠️ Report is 16 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archiver/_common.py	61.53%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9650      +/-   ##
==========================================
- Coverage   83.08%   83.05%   -0.04%     
==========================================
  Files          90       90              
  Lines       15653    15659       +6     
  Branches     2356     2359       +3     
==========================================
- Hits        13006    13005       -1     
- Misses       1887     1891       +4     
- Partials      760      763       +3     

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