import: Add Go importer.

import: Add Go importer.

This patch adds a 'guix import go' command.

* doc/guix.texi (Requirements): Mention Guile-Lib dependency.
(Invoking guix import): Document 'guix import go'.
* gnu/packages/package-management.scm (guix)[inputs, propagated-inputs]:
Add GUILE-LIB.
* guix/self.scm (compiled-guix)[guile-lib]: New variable.
[dependencies]: Add it.
(specification->package): Add "guile-lib".
* guix/build-system/go.scm (go-version->git-ref): New procedure.
* guix/import/go.scm, guix/scripts/import/go.scm, tests/go.scm: New files.
* guix/scripts/import.scm: Declare subcommand guix import go
* po/guix/POTFILES.in: Add 'guix/scripts/import/go.scm'.
* Makefile.am (MODULES): Add 'guix/import/go.scm' and
'guix/scripts/import/go.scm'.
(SCM_TESTS): Add 'tests/go.scm'.

Co-Authored-By: Helio Machado <0x2b3bfa0@gmail.com>
Co-Authored-By: Francois Joulaud <francois.joulaud@radiofrance.com>
Co-Authored-By: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Co-Authored-by: Ludovic Courtès <ludo@gnu.org>