bug#59071: guix home does not respect package outputs

bug#59071: guix home does not respect package outputs

[pronaip]

Package outputs are apparently tricky, people seem to tend to forget
they exist.  This is another case of that.

This should be a good enough reproducer:
```
(home-environment
  (packages
    (map specification->package+output
      (list "glib:bin"
            ...)))
  ...)
```
If you look at guix home describe --list-installed you'll see that all
the lines say "out" for the package output.

For a while I didn't notice this even though it **should** have broken
transmission:gui when I migrated my home config to guix home, but it
seems like the default profile did not get deleted.  This is actually
pretty confusing because guix package still continues to work and it's
not clear if there is a precendence between it and guix home and what
that precedence might be.  But that's for another bug report.

bug#59071: guix home does not respect package outputs

[Ludovic Courtès]

Hi,

pronaip@riseup.net skribis:

> This should be a good enough reproducer:
>
> ```
> (home-environment
>   (packages
>     (map specification->package+output
>       (list "glib:bin"
>             ...)))
>   ...)
> ```

‘specification->package+output’ returns two values:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (specification->package+output "glib:bin")
$38 = #<package glib@2.73.3 gnu/packages/glib.scm:445 7f0988d69b00>
$39 = "bin"
--8<---------------cut here---------------end--------------->8---

However, ‘map’ expects its first argument to return one value and thus
discards additional values:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (map specification->package+output '("glib:bin"))
$40 = (#<package glib@2.73.3 gnu/packages/glib.scm:445 7f0988d69b00>)
--8<---------------cut here---------------end--------------->8---

Instead, you have to write ‘specifications->packages’:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (specifications->packages '("glib:bin" "inkscape"))
$42 = ((#<package glib@2.73.3 gnu/packages/glib.scm:445 7f0988d69b00> "bin") (#<package inkscape@1.2.1 gnu/packages/inkscape.scm:244 7f097cf246e0> "out"))
--8<---------------cut here---------------end--------------->8---

That’s what ‘guix home import’ does now, but I noticed it’s not
documented so I’ll add it to the manual.

Thanks,
Ludo’.