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.