cmd/link: use filepath.Clean rather than path.Clean

pname already translated package path into a file path on for statement.
Use same as filepath.Clean rather than path.Clean.

Change-Id: I9782e2396313abc39ac55e3fdcec8f53bcaff84e
Reviewed-on: https://go-review.googlesource.com/c/go/+/202377


Reviewed-by: Matthew Dempsky <mdempsky@google.com>