bug#45261: New command to mark autoremovable packages

[Juri Linkov]

Like the existing command package-menu-mark-obsolete-for-deletion
bound to '~', there is a need for a similar command that would flag
for deletion all autoremovable packages.  There is the command
package-autoremove, but it deletes all packages at once,
and its selection of packages that it proposes to remove
is wrong all the time.

So after a new command will mark packages for removal,
at least this will allow manually unmark the packages that
should not be removed, leaving only those packages marked
for deletion that really were installed as dependencies.