[core] Add import hook module

[Kyle-Verhoog]

This PR introduces a modified version of wrapt's import hook mechanism that is better suited to our needs. The fundamentals stay the same the notable differences from the wrapt system are:

• hooks are no longer removed after being called
• deregister_post_import_hook is introduced to allow the removal of hooks
• logging is added if the module is already imported

Tests are also added for this new functionality.

[brettlangdon]

minor tweaks, but otherwise good to go.

[Kyle-Verhoog]

@brettlangdon this should be good to look at again when you get the chance! 😄

[brettlangdon]

minor suggestions, but otherwise this is good to go.

[brettlangdon]

Remind me, why this approach vs:

def hook():
    pass

register_post_import_hook('module_name', hook)
deregister_post_import_hook('module_name', hook)

[Kyle-Verhoog]

I think in my initial implementation I had the idea of attaching an 'enabled' property on the hooks that would be checked. The matcher would allow us to easily filter out the disabled hooks.

[brettlangdon]

ok, as long as it makes sense with the plan

[Kyle-Verhoog]

Yeah, I'm getting myself back up to speed to see if we still want to go in this direction.

[Kyle-Verhoog]

I opted to just accept the hook. If I recall correctly, our plan was to have a layer on top of this that would be meant for enabling and disabling hooks which can interface with this general api.