Consider a Go package that is defined via go-git-package, for example

    [helloweb]
    <= go-git-package
    go.importpath = lab.nexedi.com/nexedi/helloweb
    repository    = https://lab.nexedi.com/nexedi/helloweb.git

Currently, since go-git-package references ${gowork:src}, it creates
helloweb -> gowork dependency. gowork, in turn, depends on
gowork.goinstall, which gets list of things to install from ${gowork:install}.

Currently we put only plain strings into ${gowork.install}, e.g.

    [gowork]
    install =
       lab.nexedi.com/nexedi/helloweb/go/...

but for Go modules support and for properly expressing what depends on what,
we'll want in the next patch to be able to specify something like

    [gowork]
    install =
       ${helloweb:location}/go:./...

which will create helloweb ⇄ gowork cycle.

Unfortunately buildout does not detect nor report an error for such cycles, and
simply processes parts in an order, which leads to situation where e.g.
helloweb was not yet cloned, but gowork.goinstall tries to `go install` it and
complains "no such helloweb directory".

-> Fix it by leaving gowork to use by component/golang/ users, and putting
settings about where gowork directories is into underlying gowork.dir section.