When go doc is invoked with a single package name argument (e.g. go doc pkgname)
it needs to find the directory of the requested package sources in GOPATH.

GOPATH might contain directories with the same name as the requested package
that do no contain any *.go files. This change makes "go doc" ignore such
directories when looking for possible package directories.

This fixes #10882

Change-Id: Ib3d4ea69a25801c34cbe7b044de9870ba12f9aa8
Reviewed-on: https://go-review.googlesource.com/10190Reviewed-by: Rob Pike <r@golang.org>