Try avoiding loading long package description

Try avoiding loading long package description

It's a fairly expensive call and it's called on every package,
even though it's usually only used when we're interested in
a small number of packages.
Long description is currently only shown by this function
when using `apt search X --full`.

On my PC, this patch speeds up `apt list` by roughly 20%
and `apt list --installed` by 1-2%.