bpo-45020: Freeze some of the modules imported during startup. (gh-28335)

bpo-45020: Freeze some of the modules imported during startup. (gh-28335)

Doing this provides significant performance gains for runtime startup (~15% with all the imported modules frozen). We don't yet freeze all the imported modules because there are a few hiccups in the build systems we need to sort out first. (See bpo-45186 and bpo-45188.)

Note that in PR GH-28320 we added a command-line flag (-X frozen_modules=[on|off]) that allows users to opt out of (or into) using frozen modules. The default is still "off" but we will change it to "on" as soon as we can do it in a way that does not cause contributors pain.

https://bugs.python.org/issue45020