bug#54097: go importer does not honor multi-module repositories

[Björn Höfling]

Go usually has the 1 repository=1 module convention. However, it
is also allowed that one repository contains multiple go modules.

If repository "foo" contains only one module, then versions are tagged
"v1.2.3".

However, if the repository "foo" contains modules "bar" and "baz", each
in a sub-directory of "foo", the versions will be tagged with their
respective prefix, i.e.:

foo/v1.2.3
bar/v4.5.6

See here:

https://github.com/golang/go/wiki/Modules#publishing-a-release
https://github.com/golang/go/wiki/Modules#faqs--multi-module-repositories
https://stackoverflow.com/questions/64701064/golang-separate-versioning-of-multiple-modules

However, our go-importer does not honor this. The Google Cloud API
modules are structured into sub-modules, but our importer searches the
wrong tag and raises an exception:

$ ./pre-inst-env guix import go cloud.google.com/go/storage
URL FOR VERSIONS: https://proxy.golang.org/cloud.google.com/go/storage/@v/list
FETCH_GO_MOD: 
https://proxy.golang.org/cloud.google.com/go/storage/@v/v1.21.0.mod
Backtrace:
In ice-9/boot-9.scm:
  1752:10 17 (with-exception-handler _ _ #:unwind? _ # _)
In unknown file:
          16 (apply-smob/0 #<thunk 7f5b3d24f0c0>)
In ice-9/boot-9.scm:
    724:2 15 (call-with-prompt _ _ #<procedure default-prompt-handle?>)
In ice-9/eval.scm:
    619:8 14 (_ #(#(#<directory (guile-user) 7f5b3d255c80>)))
In guix/ui.scm:
   2209:7 13 (run-guix . _)
  2172:10 12 (run-guix-command _ . _)
In guix/scripts/import.scm:
   124:11 11 (guix-import . _)
In ice-9/boot-9.scm:
  1752:10 10 (with-exception-handler _ _ #:unwind? _ # _)
In guix/scripts/import/go.scm:
   116:29  9 (_)
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:
   525:18  6 (go-module->guix-package "cloud.google.com/go/storage" # ?)
In guix/git.scm:
    277:4  5 (update-cached-checkout _ #:ref _ #:recursive? _ # _ # _ ?)
   266:18  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/v1.21.0' not found


The correct git reference to look for is:

refs/tags/storage/v1.21.0

Björn

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