use exocet instead of the builtin Python module loader

[zooko]

​exocet is a replacement for the builtin Python module loader. It makes the module system no longer be a global (mutable) namespace, but instead each module has a separate namespace for the modules that it loads. This facilitates dependency injection e.g. for unit testing. It may be useful for various things. In general, the notion of eliminating a single global mutable namespace in favor of separate, narrowly-scoped namespaces sounds great!

• ​Modules in Python: the Good, the Bad, and the Ugly
• ​Introducing Exocet
• ​Exocet: A Second Look
• exocet's ​README

Over on ticket #1334, I pointed out in comment:15:ticket:1334 that exocet could facilitate blacklisting a module to test our dependencies, and davidsarah pointed out in comment:16:ticket:1334 that exocet itself requires pywin32 (oops).