x/tools/cmd/gotype: support type checking of xtest packages.
x/tools/cmd/gotype: support type checking of xtest packages.

As noted by griesemer in golang/go#18799, this doesn't address the issues
raised in golang/go#11415 because when checking an xtest package the
corresponding package is assumed to have been installed. This
however is similar to the assumptions made by go vet (raised
as an issue in golang/go#16086). So whilst not perfect, it will probably
suffice until golang/go#11415 is resolved.

Fixes golang/go#18799

Change-Id: I1ea005c402e5d6f5abddda68fee6386b0531dfba
Reviewed-on: https://go-review.googlesource.com/36992
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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