Checking with importlib.abc and importlib.util (which turned out not to have the problem) put me on the right path: the problem is the import of "imp" in importlib.__init__

What appears to be happening is that the interpreter sees the partially initialised importlib package, considers it good enough, and continues on with importing _bootstrap and machinery from source. Control eventually returns to __init__ and it overwrites the source version with the frozen version.

Changing the __init__ file to do "import _imp" instead (the same as what import_init does in the C code) was enough to fix it.

No patch yet - the actual fix is trivial, but it needs a test.