Use libapt-pkg's pkgCacheFile for a bunch of cache-related stuff inside QApt::Cache, rather than rolling our own.
This results in the nice loss of 51 lines of code.

The class will stay due to the trustCache stuff it implements (and obviously binary/API compatibility).
Whenever the need arises for a QApt2 (API break) this will likely become a thin(er) wrapper over the pkgCacheFile, with its main objective being to provide a Qt-style API. (Hopefully a trustCache solution will make it in to APT in the meanwhile, so that we and synaptic don't have to have two implementations)

But, in the meantime, less stuff to maintain/worry about breaking. :)