refactor suggestion

[hannesdelbeke]

PS: while working on #380, i noticed discover and registering a plugin directly have a lot of overlapping code.

both

• def plugins_from_module(module)
• def register_plugin(plugin)

contain

• host_is_compatible()
• version_is_compatible
• plugin_is_valid

But there are differences:

• register_plugin checks if if not hasattr(plugin, "__call__"):
• plugins_from_module doesn't

since all of these functions check if a plugin is valid, is it a bad suggestion to move them to the plugin_is_valid function?
will create a PR with a suggestion once 380 is merged in

[hannesdelbeke]

see PR #386 with suggested changes
would also give a more accurate overview of coverage.
several if statements in discover are not triggered, yet since the function has a test it is 100% covered...