Brett,

I wrote my patch thinking that the next step would be to rewrite
DirectoryImporter in Python.  If you're already working on that and
just want to skip straight from point A to point C and skip this point
B, I'm fine with that.  Basically, tell me if you want me to pursue
this further or drop it.  Are you also reimplementing the code for
builtin and frozen modules in Python?

Alexander,

The const thing: I went on a bit of a const rampage after an
especially irritating debugging session involving function side
effects.  They could be removed.

The non-subclassable aspect was an oversight.  I thought about having
a common base class but ended up creating the ModuleInfo helper class
instead.  There's still a few bits of code duplication between the two
importer classes.  16x2 lines in  *_find_module() could be pulled into
a common place, 4x2 lines of *_require_module(), and a few lines in
the various methods.  I wanted to err on the side of introducing as
few new classes as possible to minimize the general scariness of this
patch.

I don't have a Mac setup, but I think the error you're seeing is
actually an error in my test code.