The module code in cmd/go sometimes needs to know whether it is
looking at a standard package, and currently uses gc-specific code for
that. This CL moves the existing isStandardPackage code in the
go/build package, which works for both gc and gccgo, into a new
internal/goroot package so that cmd/go can call it. The changes to
cmd/go will be in a subsequent CL.

Change-Id: Ic1ce4c022a932c6b3e99fa062631577085cc6ecb
Reviewed-on: https://go-review.googlesource.com/137435
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>