ftbfs on yakkety

juju-core ftbfs on yakkety like this:

   dh_golang
Use of uninitialized value $ENV{"DH_GOPKG"} in concatenation (.) or string at /usr/share/perl5/Debian/Debhelper/Buildsystem/golang.pm line 148.
warning: "/..." matched no packages
Use of uninitialized value in split at /usr/share/perl5/Debian/Debhelper/Buildsystem/golang.pm line 150.
can't load package: package .: no buildable Go source files in /<<PKGBUILDDIR>>
go list of targets failed with code 256, at /usr/bin/dh_golang line 48.
debian/rules:26: recipe for target 'binary' failed

This is my fault; I changed dh_golang in debian to use go list and friends to compute Built-Using, rather than processing Build-Depends and the way that juju-core's rules are set up doesn't let dh_golang find the Go packages. Perhaps dh_golang shouldn't override a GOPATH set in the environment (which I think would fix this) but something that's a bit strange about the juju packaging is that it uses any installed go package in favour of the one bundled by juju, rather than just the ones it build-depends upon. This is fine on a buildd, but it seems to me that it might be a bit confusing on a developer system. lxd does this sort of thing in it's packaging: https://github.com/lxc/lxd-pkg-ubuntu/blob/dpm-yakkety/debian/rules#L50 -- would juju consider that? Then it can use a single entry GOPATH and make sure dh_golang can find it by passing --builddirectory=$whatever to dh in the default rule.

I think I'll patch dh_golang tomorrow either way...