At this point the fundamental work is done.  Here are some follow-up tasks to wrap up this issue:

* freeze the remaining stdlib modules imported during startup (os, site, codecs, encodings.*)
   + blocked by bpo-45186 and bpo-45188
* default to "on" (except if actually running out of the source tree)
   + blocked by bpo-45211 (if we want to minimize disk access)
* always default to "on" if it's a PGO build, even if running out of the source tree
* stop tracking the frozen module .h files in the repo
   + blocked by bpo-45188
* (maybe) freeze modules imported for "python -m ..." (e.g. runpy)
* (maybe) freeze a small subset of the encodings (e.g. UTF-8)
   + blocked by a valid __path__ (see below) and consequently bpo-45211

Other related follow-up tasks:

* (maybe) make frozen stdlib modules more like source modules, with __file__ and __path__ (see bpo-21736)
   + blocked by bpo-45211
   + __path__ allows us to have frozen stdlib packages with non-frozen submodules (e.g. encodings)
* (maybe) freeze other modules (e.g. commonly used modules, sysconfig)
* (maybe) use something other than linear search for looking up frozen modules (see bpo-45213)