Reduce complexity of the code in Lintian::Unpacker.

The code in Lintian::Unpacker is riddled with dense, complicated code.
It is hard to understand what is going on.  Some routines return
function references.  For an uninitiated contributor, the module is
probably impossible to edit.  Simplifies the code and keeps the same
functionality as before.
3 jobs for master in 95 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #205805
stable-bpo

01:35:52

passed #205804
testing

01:17:46

passed #205803
unstable

01:23:50