bug#15756: Cannot remove a specific package output from profile

[ludo@gnu.org (Ludovic Courtès)]

With Guix 0.4 it’s impossible to remove a single output of a package
when several of them are installed:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix package -p foo -I
make	3.82	out	/nix/store/zc7djchn80qbji4dmfvbwnw9akag0bfa-make-3.82
make	3.82	debug	/nix/store/xpw5b3kym02ra3lbaqhxqp88fqc5g7vl-make-3.82-debug
idutils	4.6	out	/nix/store/4cbh3hipf7i5aqy9fdwsb6dfla9k62sz-idutils-4.6

$ ./pre-inst-env guix package -p foo -r make:debug
nothing to be done

$ ./pre-inst-env guix package -p foo -r make
The following package will be removed:
   make-3.82	/nix/store/xpw5b3kym02ra3lbaqhxqp88fqc5g7vl-make-3.82-debug

1 package in profile

$ ./pre-inst-env guix package -p foo -I
idutils	4.6	out	/nix/store/4cbh3hipf7i5aqy9fdwsb6dfla9k62sz-idutils-4.6
--8<---------------cut here---------------end--------------->8---

Ludo’.