Fixed warnings about not removable non-empty folders in apt_clean()

[Yrrsinn]

freight-cache produces multiple lines warnings (proportional to the size of cache): find: cannot delete [...]/cache/pool/[...]: Directory not empty

The behavior is correct. This directories should only be removed, when they are empty after removing old packages. Thus the warnings are erroneous.

[domcleal]

:+1: looks good otherwise.

@mmoll you mentioned about Linux/GNUisms the other day, is -empty one and should we be bothered about it at this stage (i.e. are there others)?

[mmoll]

@domcleal this is perfectly fine, as there are already a whole lot of these and the only way at the moment is to install findutils and coreutils as packages and use the respective binaries with g/gnu prefix (which is also perfectly normal to do for an FreeBSD package to patch these prefixes in).

@Yrrsinn could you squash the commits together into one and force-push?

[mmoll]

:+1:

[Yrrsinn]

@mmoll squashed and rebased ~ ready on my side

[mattock]

:+1:

[domcleal]

Thanks @Yrrsinn!