Dependency paths on targets that generate subrepos aren't surfaced

[peterebden]

An example is probably worth a thousand (or at least several hundred) words:

$ plz query somepath //src:please //third_party/go:_golang.org_x_term#dl
Couldn't find any dependency path between //src:please and //third_party/go:_golang.org_x_term#dl

Clearly there is a dependency here in a real sense, since Please uses x/term and that #dl target is the thing that downloads the files for it. Ideally queries should surface this relationship.
(The same is true for //third_party/go:golang.org_x_term)

This is an issue for getting go_repo targets labelled correctly (see please-build/go-rules#192)