Merge ppa-dev-tools:add_suite_module into ppa-dev-tools:main

Splits out and fleshes out the Suite class from the Repository class. This will house the major logic for the rdepends testing functionality, but this initial branch just gets the basic structure in place for accessing the packages within an archive.

A Suite is defined as a particular pocket of a release, so that's like lunar-proposed or focal-backports. Internally it is further divided by component, IOW main, restricted, universe, multiverse. The Suite class encapsulates the logic for dealing with all these divisions.

As usual, the unit tests for Repository and Suite can be run via:

  $ pytest-3 tests/test_repository.py tests/test_suite.py

For static testing, I've been using the check-scripts tool from ubuntu-helpers:

  $ check-scripts ./tests/test_suite.py ./tests/test_repository.py ./ppa/repository.py ./ppa/suite.py

Last, there's also a smoketest for the Suite class, which can be used if you have a local mirror of the Apt repository (I made mine using the `apt-mirror` utility). Here's what the output looks like on my system:

  $ python3 -m ppa.suite
lunar-proposed
  series: lunar
  pocket: proposed
  components: main
  architectures: amd64, arm64, armhf, i386, ppc64el, riscv64, s390x
  sources: (203 items)
    0 adsys
    1 apt
    2 at-spi2-core
    [...]
    200 xorg-server
    201 xz-utils
    202 zip
  binaries: (931 items)
    0 adsys:amd64
    1 apt:amd64
    2 apt-doc:amd64
    [...]
    928 xz-utils:amd64
    929 zip:amd64
    930 zstd:amd64
lunar
  series: lunar
  pocket: release
  components: main
  architectures: amd64, arm64, armhf, i386, ppc64el, riscv64, s390x
  sources: (2369 items)
    0 aalib
    1 abseil
    2 accountsservice
    [...]
    2366 zsh
    2367 zsys
    2368 zvmcloudconnector
  binaries: (6045 items)
    0 accountsservice:amd64
    1 acct:amd64
    2 acl:amd64
    [...]
    6042 zstd:amd64
    6043 zsys:amd64
    6044 zvmcloudconnector-common:amd64