Reimplement pacman cache cleaning the right way

Partial cache cleaning was eliminated in a previous commit because it relied
on package naming conventions. Re-add it the correct way- we actually open
up each package in the cache and get a name and version out of it. If the
name and version match that of an installed package, keep it. If the package
is not installed or the version does not match the locally-installed version,
get rid of it.

This can easily be modified if some other heuristic of keeping and removing
packages is desired, or if we should clean out the cache dir of any files
that are not packages, etc.

The biggest current problem with this new approach- speed. Here is one run
on my local machine, going from 1643 to 729 packages in the cache (753 in
the local DB):
real    4m25.829s
user    3m22.527s
sys     0m6.713s

This is likely best addressed by the package loading scheme, which may be
loading the entirety of each package archive, which is a waste when we only
need the .PKGINFO file read.

Signed-off-by: Dan McGee <dan@archlinux.org>

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