Rework plugin infrastructure to nix side-effects

This reworks the plugin infrastructure so as to remove module-loading
side-effects which were making things a pain in the ass to test.

With the new system, there's no auto-registering meta class. Instead
plugins do whatever they want and then specify a hooks dict that maps
hook names to callables for the things they're tying into. The most
common one (and the only one we've implemented so far) is "setup".

This also simplifies the sampleplugin a little by moving the code
to __init__.py.

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