We've frozen most of the stdlib modules imported during "python -c
pass" [1][2], to make startup a bit faster.  Import of those modules
is controlled by "-X frozen_modules=[on|off]".  Currently it defaults
to "off" but we'd like to default to "on".  The blocker is the impact
on contributors.  I expect many will make changes to a stdlib module
and then puzzle over why those changes aren't getting used.  That's an
annoyance we can avoid, which is the point of this thread.

Possible solutions:

1. always default to "on" (the annoyance for contributors isn't big enough?)
2. default to "on" if it's a PGO build (and "off" otherwise)
3. default to "on" unless running from the source tree

Thoughts?