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/+/202377Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/link/internal/ld/ld.go

@@ -148,7 +148,7 @@ func findlib(ctxt *Link, lib string) (string, bool) {
 				}
 			}
 		}
-		pname = path.Clean(pname)
+		pname = filepath.Clean(pname)
 	}
 
 	return pname, isshlib