code.

This should make the Linux distros happy as it is now easier to leave
importlib's tests out of their base Python distribution.

Prevent test_inspect from keeping alive a ton of frames and local variables by way of a global variable keeping a reference to a traceback.
Should fix some buildbot failures.

Most of the import sequence now uses per-module locks rather than the
global import lock, eliminating well-known issues with threads and imports.