godoc: hide vendored packages from /pkg/ as we do with internal packages
godoc: hide vendored packages from /pkg/ as we do with internal packages

Such packages may be viewed by adding ?m=all to the URL.

Fixes golang/go#13929

Change-Id: Id0ac48bc2937b5bec722b95b9a690d48d065ea6b
Reviewed-on: https://go-review.googlesource.com/18554
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,syreclabs/go-tools,cmars/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,cmars/tools,golang/tools,cmars/tools,syreclabs/go-tools,cmars/tools