Simplifying dh-golang usage

[Guillem Jover <gjover@sipwise.com>]

Hi!

I've been slightly annoyed by the boilerplate required when using
dh-golang, so I've prepared several changes to make it so that in
place of having to use:

  %:
  <TAB>dh $@ --builddirectory=_build --buildsystem=golang --with=golang

the following (in most cases) can be used instead:

  %:
  <TAB>dh $@

The first part to remove --with=golang is now in sid, by providing the
dh-sequence-golang, which can be build-depended on instead of dh-golang.

The next part to remove the need for --buildsystem=golang has two
sub-parts, one sitting in dh-golang git HEAD to add buildsystem
autodetection, and the other is on debhelper (#981106) to add «golang»
to its third-party list of supported buildsystems.

The last part would be to make the default directory for the golang
buildsystem (starting with dh compatibility level 14) be «_build»,
which seems to be the prevalent convention in the golang team(?), so that
the --builddirectory option can be omitted. I've not yet pushed that to
git, as I'd like a few more opinions on this.

Thanks,
Guillem