In Go world development workflow is organized around so-called workspace
where multiple packages can be installed / worked on etc. The following
page describes Go workspaces:

https://golang.org/doc/code.html

A new [gowork] section and infrastructure around it is introduced.
Quoting code:

    # gowork is a top-level section representing workspace
    #
    # users should add `install` field to [gowork] to describe packages they want to
    # be installed (+ automatically their dependencies are installed too). e.g.
    #
    #   [gowork]
    #   install =
    #       lab.nexedi.com/kirr/neo/go/...  \
    #       github.com/pkg/profile          \
    #       golang.org/x/perf/cmd/benchstat

The way it all works inside is:

- gowork organizes to create go.work/ directory in buildout root
- inside it creates env.sh which when sources in shell adjusts all paths
  so that appropriate go compiler is in path, GOPATH is also set
  appropriately, etc - in other words everything needs for using/working
  on this workspace is setup in the environment.
- in actual user software profile the list of Go projects which needs to
  be installed by SlapOS has to be listed. The [gowork] machinery takes
  care about git-cloning these projects and `go install`s them after.
- by SlapOS design builds have to be reproducible and so every component
  state/version has to be fixed.

  A convenient helper (gowork-snapshot) to get snapshot of git go
  packages installed with their exact revisions is added. This should be
  used to automatically (re-)generate list of go git repositories &
  their pinning for a workspace.

For a new Go project that needs to be slaposified the workflow should be:

- first get a project into working state via any convenient way (`go get`, etc.)
- generate list of go packages & their pinning with gowork-snapshot
- extend golang/buildout.cfg and in project software profile add

	[gowork]
  	install = your-top-level-packages

  as it was outlined above.

P.S.

[golang] is removed because

1. it is not used anywhere in slapos.git tree, and
2. the canonical way to work on go projects from now on will be to use
   [gowork]. There latest go version is preconfigured, but users can
   override it to some particular other version in their projects as they
   need.

/reviewed-on	 nexedi/slapos!242
/also-needed-for nexedi/slapos!243