Confusing warning message and help information on the go plugin

[Didier Roche-Tolomelli]

If a part is using the go plugin, we can see this warning (in yellow) during a snapcraft build:
"Please consider setting `go-importpath` for the 'face-detection-backend' part"

However, snapcraft help go reports:
"
    - go-importpath:
      (string)
      This entry tells the checked out `source` to live within a certain path
      within `GOPATH`.
      This is not needed and does not affect `go-packages`.
"

The "This is not needed" and printing systematically a warning is confusing. As for most of projects, we don't need go-importpath, I would suggest removing the warning.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: snapcraft 2.14
ProcVersionSignature: Ubuntu 4.4.0-34.53-generic 4.4.15
Uname: Linux 4.4.0-34-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Aug 17 08:57:11 2016
InstallationDate: Installed on 2012-05-28 (1541 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
PackageArchitecture: all
SourcePackage: snapcraft
UpgradeStatus: Upgraded to xenial on 2014-10-31 (655 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Sergio Schvezov (sergiusens)]

go vendor is the way forward and solution