bug#27822: 26.0.50; package-autoremove and 'package-directory-list

[Yuri D'Elia <wavexx@thregr.org>]

package-autoremove shouldn't attempt to remove external packages
(additional packages detected via in package-directory-list).

Debian now includes several copies of elpa packages in the repository.
The directory including those are added to 'package-directory-list.

I also manage some personal packages locally, and I also add the
directory into 'package'directory-list.

These packages are shown as "external" by "list-packages", which is
correct. They are obviously not directly installed via package-install,
and cannot be autoremoved either.

package-autoremove however will still attempt to remove them. My feeling
is that any package which is not directly in 'package-user-dir shouldn't
be autoremoved.