* guix/scripts/package.scm (read/eval-package-expression): New
procedure.
(show-help): Add `-e'.
(%options): Likewise.
(guix-package)[process-actions]: Handle ('install . p) pairs, where P
is a package.
* tests/guix-package.sh: Add `boot_make_drv'. Use `-i $boot_make_drv'
once, and then use `-e $boot_make'.
* doc/guix.texi (Invoking guix package): Document `-e'.