apt should (optionally) delete cached debs once installed

Binary package hint: apt

It would be really useful to have an option to apt to make it never cache packages. I know this can be done manually by running apt-get clean, but
it's a pity that this can't be automated by a setting in the config file. By default, this can waste quite a lot of one's root partition.

This is a particular problem if:

a)The disk is very small. For example, I run ubuntu + KDE + amarok on a 1GB compact flash card, in order to have a totally silent system for music (the oggs are accessed via sshfs).

b)The installer runs out of disk space during the install. This can happen even when installing ubuntu-minimal. I think the installer needs to check for small disks, and run apt-get clean periodically during the install.

Keeping cached copies of debs which are already installed doesn't seem to be a very useful behaviour (unless one has a very slow internet connection, or is installing to multiple machines, or has vast amouts of disk space to waste). Small root partitions are quite common, in several cases:
 * The hardware is old. 6GB disks aren't that uncommon!
 * The hardware is specialised (eg CF card or USB-key)
 * The user is testing, and doesn't want to allocate much space to ubuntu.
 * / is "only" 10GB, and starts filling up rather fast!

My suggestions are:

1)Add an option to apt, specified in apt.conf to always remove debs from the cache once they are installed. (so that the user doesn't have to keep remembering to run apt-get clean)

2)Make the installer aware when it is running out of disk space.