guix package: Add '--export-manifest'.

* guix/scripts/package.scm (export-manifest): New procedure.
(show-help, %options): Add '--export-manifest'.
(process-query): Honor it.
* guix/build/profiles.scm (build-profile): Mention it.
* tests/guix-package.sh: Test it.
* doc/guix.texi (Invoking guix package): Document it.