fix: concurrent downloads again

[qryxip]

Fixup for eb51646 and effd7a7.

Reason

The following code shows PermissionDenied on Windows, and DirectoryNotEmpty on Linux. I thought just checking the destination exists is simple and reliable.

let tmpdir = ::tempfile::tempdir()?;
let tmpdir = tmpdir.path();

std::fs::create_dir(tmpdir.join("dir1"))?;
std::fs::write(tmpdir.join("dir1").join("a.txt"), b"")?;

std::fs::create_dir(tmpdir.join("dir2"))?;
std::fs::write(tmpdir.join("dir2").join("a.txt"), b"")?;

let err = std::fs::rename(tmpdir.join("dir1"), tmpdir.join("dir2")).unwrap_err();
dbg!(err.kind());

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.02%. Comparing base (effd7a7) to head (471ddc1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #323   +/-   ##
=======================================
  Coverage   51.02%   51.02%           
=======================================
  Files          36       36           
  Lines        2677     2677           
=======================================
  Hits         1366     1366           
  Misses       1311     1311           

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