bug#55139: package-location is wrong for transformed packages

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hi Guix!

--8<---------------cut here---------------start------------->8---
> ,import (gnu packages) (guix packages)
> (car (find-packages-by-name "python2-pyalsaaudio"))
$1 = #<package python2-pyalsaaudio@0.8.4 gnu/packages/audio.scm:4327 7f2b76fab160>
> (package-definition-location-code package $1)
$2 = #f
> (package-definition-location $2)
$3 = #<<location> file: "gnu/packages/audio.scm" line: 4327 column: 2>
--8<---------------cut here---------------end--------------->8---

This is wrong though, and gives the location of the parent package (the
one it inherited from).  This seems to be made on purpose (?) given the
code in (gnu build-system python):

--8<---------------cut here---------------start------------->8---
(package/inherit p
        (location (package-location p))

        ^ Here, we explicitly set the location to that of its parent.
        
        (name (let ((name (package-name p)))
                (string-append new-prefix
                               (if (string-prefix? old-prefix name)
                                   (substring name
                                              (string-length old-prefix))
                                   name))))
        (arguments
         (let ((python (if (promise? python)
                           (force python)
                           python)))
           (ensure-keyword-arguments (package-arguments p)
                                     `(#:python ,python)))))
--8<---------------cut here---------------end--------------->8---

Another related issue for package-with-python2 packages:

--8<---------------cut here---------------start------------->8---
> (package-definition-location-code $1)
$4 = #f
--8<---------------cut here---------------end--------------->8---

I was expecting to use this to get rid of all leaf Python 2 packages,
but it seems that may not work (as easily as I'd hoped).

Thanks,

Maxim