bpo-30339, bpo-33913:

* Increase timeout from 10 seconds to 1 minute in
  test_source_main_skipped_in_children source of
  test_multiprocessing_main_handling.
* Replace time.time() with time.monotonic().
* On timeout, include the duration in the error message.

* Trivial cleanups following bpo-31370

* Also cleanup the "importlib._bootstrap_external" module

test_multiprocessing_main_handling: increase the test_source timeout
from 10 seconds to 60 seconds, since the test fails randomly on busy
buildbots.

Patch by Christie Wilson.

implementation is broken (ex: skipped on FreeBSD 6.4).

This should appease the OpenIndiana buildbot.

Also lengthened the worst case timeout to try to eliminate
the inconsistent failure on one of the Windows 7 buildbots.

Thanks to Christian Heimes for noting the buildbot failures and
to Zachary Ware for providing the patch to make the new tests
play nice with both other platforms and unittest test discovery

- handles main files without a suffix
- handles main submodules properly
- adds test cases for the various kinds of __main__