bug#36637: Package buffer should show where (which archive) a package is installed from

[Stefan Kangas <stefan@marxist.se>]

It would be good if there was an indication in the "*Packages*" buffer of where
(what archive) you have installed a package from.

I have added both MELPA and MELPA Stable to `package-archives':

    (add-to-list 'package-archives
                 '("melpa" . "https://melpa.org/packages/") t)
    (add-to-list 'package-archives
                 '("melpa-stable" . "https://stable.melpa.org/packages/") t)

I sometimes install packages from gnu (ELPA), sometimes from melpa and sometimes
from melpa-stable.

The package "ivy" is available from all three archives.  After installing the
package from gnu, when I run M-x package-list-packages, I see in the "*Package*"
buffer the following two entries (although not next to each other):

  ivy                20190709.740  available  melpa      Incremental
Vertical completYon
  ivy                0.11.0        installed             Incremental
Vertical completYon

From this, I can figure out that I have not installed it from melpa (its status
is "available").  But I cannot easily know if I installed it from gnu or from
melpa-stable, or if it's available from only one or from both of them.

In this case, the versions on melpa-stable and gnu matches.  (Or, at least, the
version number does.)  But this just happens to be the case here.  Often, the
package does not even exist on gnu, and I wouldn't know that from the above.

It would be good if the second of the above two lines would be this instead:

  ivy                0.11.0        installed  gnu        Incremental
Vertical completYon

It would be even better if the describe-package buffer would also would show the
same information.

Thanks,
Stefan Kangas