import: go: Append version to symbol name in the pinned version mode.

This allows importing packages with complicated version specific dependency
chains without the package symbol names colliding.

* doc/guix.texi (Invoking guix import): Document the --pin-versions option.
Mention that a specific version can be imported.  Remove the experimental
warning.
* guix/import/go.scm (go-module->guix-package-name)[version]: Add optional
argument.  Rewrite the character translation in terms of string-map.
(go-module->guix-package): Conditionally use dependencies whose symbol include
their version, based no the value of the PIN-VERSIONS? argument.
* guix/import/utils.scm (package->definition): Add a new case where the full
version string is appended to the package symbol.
* guix/scripts/import.scm (guix-import): Correctly print forms starting
with '(define-public [...]'.
* guix/scripts/import/go.scm (guix-import-go): Conditionally include the
version in the package symbols defined.