Previously format argument was detected via scanning func type args.
This didn't work when func type couldn't be determined if the func
is declared in the external package. Fall back to scanning for
the first string call argument in this case.

Fixes #14754

Change-Id: I571cc29684cc641bc87882002ef474cf1481e9e2
Reviewed-on: https://go-review.googlesource.com/21023
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>