Not sure how they got there, but Django doesn't use Jinja without
explicit configuration.

This matches a tweak made in 8765f761c995c31c9bff6287d881f0e9b5d0cfc5

There is only so much trackpad-driven yEd work one man can handle

I took the liberty of renaming ModuleFinder.STDLIB_DIRS to
_STDLIB_PATHS, since it felt like an implementation detail that
shouldn't be baked into a public API and stdlib can also be imported
from e.g. a zip file.

I also changed it to a set to handle any duplicates.

Fixes #86

The solution was that Mitogen's loader should emulate the behaviour of
ansible.executor.module_common, which restricts dependency scanning to
the ansible.module_utils namespace.

Doesn't need to go via defer() since it's always running on the broker
thread.

Using the same test as in 7af97c0365d86ec851c56d2d3736be2cc281af29,
transmitted wire bytes drops from 135,531 to 133,071 (-1.81%), while
received drops from 21,073 to 14,775 (-30%).

Combined, both changes shave 13,914 bytes (-8.6%) off aggregate
bandwidth usage.

Make it configurable as compression hurts in some scenarios.