bug#63001: Go importer doesn't know MODULE/vX.Y version tags

["Timo Wilken" <guix@twilken.net>]

Some Golang repositories apparently use a tagging scheme where each
submodule in the repo gets its own tags.

In this case, the example is cloud.google.com/go and its submodules
(hosted at https://github.com/googleapis/google-cloud-go).

For example, for the cloud.google.com/go/longrunning submodule at
version 0.4.1, the Go importer should look for a tag called
"refs/tags/longrunning/v0.4.1" in the Git repository.

#+begin_example
$ guix import go -r cloud.google.com/go/longrunning
Backtrace:
          19 (primitive-load "/home/timo/.config/guix/current/bin/gu…")
In guix/ui.scm:
   2300:7 18 (run-guix . _)
  2263:10 17 (run-guix-command _ . _)
In guix/scripts/import.scm:
    89:11 16 (guix-import . _)
In ice-9/boot-9.scm:
  1752:10 15 (with-exception-handler _ _ #:unwind? _ # _)
In guix/scripts/import/go.scm:
   114:22 14 (_)
In guix/import/utils.scm:
   638:27 13 (recursive-import _ #:repo->guix-package _ #:guix-name . #)
   630:33 12 (lookup-node "cloud.google.com/go/longrunning" #f)
In guix/memoization.scm:
     98:0 11 (mproc "cloud.google.com/go/longrunning" # #<procedure…> …)
In unknown file:
          10 (_ #<procedure 7fb3e173c820 at guix/memoization.scm:17…> …)
In guix/import/go.scm:
   685:10  9 (_ _ #:version _ #:repo _)
In ice-9/exceptions.scm:
   406:15  8 (go-module->guix-package* . _)
In ice-9/boot-9.scm:
  1752:10  7 (with-exception-handler _ _ #:unwind? _ # _)
In guix/import/go.scm:
   511:19  6 (go-module->guix-package "cloud.google.com/go/longrunn…" …)
In guix/git.scm:
    291:4  5 (update-cached-checkout _ #:ref _ #:recursive? _ # _ # _ …)
   277:19  4 (resolve _)
In git/reference.scm:
     60:8  3 (_ _ _)
In git/bindings.scm:
     77:2  2 (raise-git-error _)
In ice-9/boot-9.scm:
  1685:16  1 (raise-exception _ #:continuable? _)
  1683:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1683:16: In procedure raise-exception:
Git error: reference 'refs/tags/v0.4.1' not found
#+end_example

The same is true for other modules, e.g.
github.com/Azure/azure-sdk-for-go/sdk/internal, where the tag is
called "refs/tags/sdk/internal/v1.3.0" instead:

#+begin_example
$ guix import go -r github.com/Azure/azure-sdk-for-go/sdk/internal
Backtrace:
          19 (primitive-load "/home/timo/.config/guix/current/bin/gu…")
In guix/ui.scm:
   2300:7 18 (run-guix . _)
  2263:10 17 (run-guix-command _ . _)
In guix/scripts/import.scm:
    89:11 16 (guix-import . _)
In ice-9/boot-9.scm:
  1752:10 15 (with-exception-handler _ _ #:unwind? _ # _)
In guix/scripts/import/go.scm:
   114:22 14 (_)
In guix/import/utils.scm:
   638:27 13 (recursive-import _ #:repo->guix-package _ #:guix-name . #)
   630:33 12 (lookup-node "github.com/Azure/azure-sdk-for-go/sdk/in…" …)
In guix/memoization.scm:
     98:0 11 (mproc "github.com/Azure/azure-sdk-for-go/sdk/internal" …)
In unknown file:
          10 (_ #<procedure 7fa449ec0300 at guix/memoization.scm:17…> …)
In guix/import/go.scm:
   685:10  9 (_ _ #:version _ #:repo _)
In ice-9/exceptions.scm:
   406:15  8 (go-module->guix-package* . _)
In ice-9/boot-9.scm:
  1752:10  7 (with-exception-handler _ _ #:unwind? _ # _)
In guix/import/go.scm:
   511:19  6 (go-module->guix-package "github.com/Azure/azure-sdk-f…" …)
In guix/git.scm:
    291:4  5 (update-cached-checkout _ #:ref _ #:recursive? _ # _ # _ …)
   277:19  4 (resolve _)
In git/reference.scm:
     60:8  3 (_ _ _)
In git/bindings.scm:
     77:2  2 (raise-git-error _)
In ice-9/boot-9.scm:
  1685:16  1 (raise-exception _ #:continuable? _)
  1683:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1683:16: In procedure raise-exception:
Git error: reference 'refs/tags/v1.3.0' not found
#+end_example