emacs: Speed up starting the REPL.

Delay initializing variables.

* emacs/guix-main.scm: (%packages): Rename to...
(%package-vhash): ... this.  Delay setting the value.
(package-vhash): New procedure (wrapper for '%package-vhash').
(%package-table): Delay setting the value.
(package-table): New procedure (wrapper for '%package-table').
(package-by-address, packages-by-name+version): Use wrappers.