Fixes #37377 - Add interop method for find_module

[sbernhard]

In python2, only module "imp" exists.
In python3 the importlib module exists to check if a module exists.

[ianballou]

Mind if we get a redmine on here?

[ekohl]

Which supported platforms still have Python 2? Isn't that jut EL 6 & 7? Or is there also a SLES with Python 2?

I was also thinking about refactoring the whole code to something like this:

MODULES = {
    'dnf': 'katello.tracer.dnf',
    'yum': 'katello.tracer.yum',
    'apt': 'katello.tracer.deb',
    'zypp_plugin': 'katello.tracer.zypper',
}

for mod, collector in MODULES.items():
    if importlib.util.find_spec(mod):
        importlib.import_module(collector, 'collect_apps')
        break
else:
    def collect_apps():
        raise NotImplementedError("Couldn't detect package manager. Failed to query affected apps!")

There are several problems with this:

• importlib isn't available everywhere
• Only in modern Python 3 is the order of a dictionary guaranteed
• It assumes a katello.tracer.yum is created

The first you could capture with the compatibility code, similar to what you wrote. The second could be resolved by renaming deb and zypper to match the top level name and use a simple list. The last is simple.

If only EL 6 & 7 are on Python 2, then you can also conclude that importlib's absence implies you have yum.