rpm packaging 2.0

[TomasTomecek]

This is a mirror of https://bugzilla.redhat.com/show_bug.cgi?id=1677664

I completely agree with @lslebodn, we shouldn't force our users to install all the possible deps of conu. Therefore I propose this:

Short term:

• Do what Lukas suggests
  • Use weak dependencies for tools required by all the backends
    • systemd-nspawn
    • docker
    • podman
    • anything else?
  • The requirement is that conu can be imported and initiated even with all the weak deps are not installed
  • There should be sensible error messages and guidance if people want to use e.g. podman backend and podman is not installed

Long term:

• Package every backend as a subpackage
• Use hard dependencies in the subpackages
• Make sure conu works even when a backend is not installed (mainly conu/__init__.py)