cmd/link: use filepath.Clean rather than path.Clean
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 <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

CAFxX/go,CAFxX/go,CAFxX/go,golang/go,akutz/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,akutz/go,nawawi/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,golang/go,akutz/go,nawawi/go,akutz/go,akutz/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,golang/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go