import: fix the repo-scope checks on Windows

Run against a Windows runner, two checks misbehaved:

• The Cursor collision walk compared directory listings (long names) against a target spelled with 8.3 short names (RUNNER~1), so it found no collision and let the import through. The target is now resolved to its canonical spelling first.
• The relative-cwd guard used filepath.IsAbs, which on Windows rejects drive-less rooted paths such as \work\repo. It now accepts any rooted path and still rejects relative and drive-relative ("C:foo") ones; a drive-less path never matches a drive-qualified repo root anyway.