The PR has more work to do, but the overall layout/changes are more or less there, so happy to discuss feedback/etc.

Obviously there are a lot of edge cases here, but they seem to be mostly tested already. And I think we're early enough in alpha to find any major issues (or absorb any necessary minor changes - seems like trailing slashes might change on some paths).

There are also some changes/hacks into the new frozen module support, so that I can freeze getpath.py without turning it into a module. I really just want to execute the bytecode - no reason for any of its contents to stick around - and this works out pretty neatly. But if the changes to frozen modules seem off then maybe we can split it into totally separate freezing support?