If a directory in GOPATH is unreadable, we should keep looking for other
packages. Otherwise we can give the misleading error "no buildable Go
source files".

Fixes #16240

Change-Id: I38e1037f56ec463d3c141f0508fb74211cb90f13
Reviewed-on: https://go-review.googlesource.com/31713
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>