Reinstate caching of file hashes in apt-ftparchive

Reinstate caching of file hashes in apt-ftparchive

Check for cached hash entries to determine which (if any) hash types
need to be generated for the current file.
In 1.0.9, each hash type was handled by a separate method, each of
which checked the cache. It looks like when these code paths were
unified (in a311fb96b84757ef8628e6a754232614a53b7891) the cache
checks were not incorporated into the new method.

(cherry picked from commit 51018e947ab1df3ddba5d7a84ed2284d599d8a12)

Pull request Debian/apt#16

Closes: #806924