self: Produce a complete package with the 'guix' command.

self: Produce a complete package with the 'guix' command.

* guix/self.scm (guix-command): New procedure.
(compiled-guix): Add #:pull-version parameter.
[command, package]: New variables.
Honor PULL-VERSION.
(guix-derivation): Add #:pull-version and pass it to 'compiled-guix'.
* build-aux/build-self.scm (build-program): Add #:pull-version
parameter.  Pass it to 'guix-derivation'.
(build): Add #:pull-version and pass it to 'build-program'.
* build-aux/compile-as-derivation.scm: Pass #:pull-version to BUILD.