Oliver Smith authored Nov 15, 2018 and Martijn Braam committed Dec 01, 2018

Add a new action that lists all aports, for which no binary packages
exist. Only list packages that can be built for the relevant arch
(specified with --arch). This works recursively: when a package can be
built for a certain arch, but one of its dependencies
(or their depends) can not be built for that arch, then don't list it.

This action will be used for the new sr.ht based build infrastructure,
to figure out which packages need to be built ahead of time (so we can
trigger each of them as single build job). Determining the order of the
packages to be built is not determined with pmbootstrap, the serverside
code of build.postmarketos.org takes care of that.

For testing purposes, a single package can also be specified and the
action will list if it can be built for that arch with its
dependencies, and what needs to be built exactly.

Add pmb/helpers/package.py to hold functions that work on both pmaports
and (binary package) repos - in contrary to the existing
pmb/helpers/pmaports.py (see previous commit) and pmb/helpers/repo.py,
which only work with one of those.

Refactoring:
* pmb/helpers/pmaports.py: add a get_list() function, which lists all
  aports and use it instead of writing the same glob loop over and over
* add pmb.helpers.pmaports.get(), which finds an APKBUILD and parses it
  in one step.
* rename pmb.build._package.check_arch to ...check_arch_abort to
  distinguish it from the other check_arch function