guix package: Fix handling of `-e'.

This fixes a bug whereby a command like

  guix package -e '(@@ (gnu packages base) gnu-make-boot0)'

would only succeed when the outputs of that package are available, and
otherwise fail with "/nix/store/... is not valid".

* guix/scripts/package.scm (guix-package)[process-action](package->tuple):
  Leave P as is in the result.  Move `package->tuple' call from INSTALL*
  to INSTALL.