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.

For the 52 submodules of ansible.modules.system, this produced a 1602
byte pkg_present list. After stripping it becomes 406 bytes, and the
entire LOAD_MODULE size drops from 1988 bytes to 792 bytes (-60%).

For the 68 submodules of ansible.module_utils, 1902 bytes pkg_present
becomes 474 bytes (-75%), and LOAD_MODULE size drops from 2867 bytes to
1439 bytes (-49%).

In a simple test running Ansible's "setup" module followed by its "apt"
module, wire bytes sent drops from 140,357 to 135,531 (-3.4%).