Sorry I should have thought of trying -v. The output (included below) doesn't seem to offer many hints, though. runpy.pyc is in python36.zip, I checked that.

I'll see if I can find a machine without Python installed to test that case.

>.\python.exe -v .\test.pyz
import _frozen_importlib # frozen
import _imp # builtin
import sys # builtin
import '_warnings' # <class '_frozen_importlib.BuiltinImporter'>
import '_thread' # <class '_frozen_importlib.BuiltinImporter'>
import '_weakref' # <class '_frozen_importlib.BuiltinImporter'>
import '_frozen_importlib_external' # <class '_frozen_importlib.FrozenImporter'>
import '_io' # <class '_frozen_importlib.BuiltinImporter'>
import 'marshal' # <class '_frozen_importlib.BuiltinImporter'>
import 'nt' # <class '_frozen_importlib.BuiltinImporter'>
import _thread # previously loaded ('_thread')
import '_thread' # <class '_frozen_importlib.BuiltinImporter'>
import _weakref # previously loaded ('_weakref')
import '_weakref' # <class '_frozen_importlib.BuiltinImporter'>
import 'winreg' # <class '_frozen_importlib.BuiltinImporter'>
# installing zipimport hook
import 'zipimport' # <class '_frozen_importlib.BuiltinImporter'>
# installed zipimport hook
# zipimport: found 609 names in 'C:\\Work\\Scratch\\test\\python36.zip'
import 'zlib' # <class '_frozen_importlib.BuiltinImporter'>
# zipimport: zlib available
# zipimport: zlib available
# zipimport: zlib available
# zipimport: zlib available
import '_codecs' # <class '_frozen_importlib.BuiltinImporter'>
import codecs # loaded from Zip C:\Work\Scratch\test\python36.zip\codecs.pyc
# zipimport: zlib available
# zipimport: zlib available
import encodings.aliases # loaded from Zip C:\Work\Scratch\test\python36.zip\encodings\aliases.pyc
import encodings # loaded from Zip C:\Work\Scratch\test\python36.zip\encodings\__init__.pyc
# zipimport: zlib available
# zipimport: zlib available
import encodings.utf_8 # loaded from Zip C:\Work\Scratch\test\python36.zip\encodings\utf_8.pyc
import '_signal' # <class '_frozen_importlib.BuiltinImporter'>
# zipimport: zlib available
# zipimport: zlib available
import encodings.latin_1 # loaded from Zip C:\Work\Scratch\test\python36.zip\encodings\latin_1.pyc
# zipimport: zlib available
# zipimport: zlib available
# zipimport: zlib available
# zipimport: zlib available
# zipimport: zlib available
# zipimport: zlib available
import _weakrefset # loaded from Zip C:\Work\Scratch\test\python36.zip\_weakrefset.pyc
import abc # loaded from Zip C:\Work\Scratch\test\python36.zip\abc.pyc
import io # loaded from Zip C:\Work\Scratch\test\python36.zip\io.pyc
Python 3.6.0 (v3.6.0:41df79263a11, Dec 23 2016, 08:06:12) [MSC v.1900 64 bit (AMD64)] on win32
# zipimport: zlib available
# zipimport: zlib available
import encodings.cp437 # loaded from Zip C:\Work\Scratch\test\python36.zip\encodings\cp437.pyc
# zipimport: found 1 names in '.\\test.pyz'
Could not import runpy module
Traceback (most recent call last):
  File "<frozen importlib._bootstrap>", line 961, in _find_and_load
  File "<frozen importlib._bootstrap>", line 948, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'runpy'
# clear builtins._
# clear sys.path
# clear sys.argv
# clear sys.ps1
# clear sys.ps2
# clear sys.last_type
# clear sys.last_value
# clear sys.last_traceback
# clear sys.path_hooks
# clear sys.path_importer_cache
# clear sys.meta_path
# clear sys.__interactivehook__
# clear sys.flags
# clear sys.float_info
# restore sys.stdin
# restore sys.stdout
# restore sys.stderr
# cleanup[2] removing builtins
# cleanup[2] removing sys
# cleanup[2] removing _frozen_importlib
# cleanup[2] removing _imp
# cleanup[2] removing _warnings
# cleanup[2] removing _thread
# cleanup[2] removing _weakref
# cleanup[2] removing _frozen_importlib_external
# cleanup[2] removing _io
# cleanup[2] removing marshal
# cleanup[2] removing nt
# cleanup[2] removing winreg
# cleanup[2] removing zipimport
# cleanup[2] removing zlib
# cleanup[2] removing encodings
# destroy encodings
# cleanup[2] removing codecs
# cleanup[2] removing _codecs
# cleanup[2] removing encodings.aliases
# cleanup[2] removing encodings.utf_8
# cleanup[2] removing _signal
# cleanup[2] removing __main__
# destroy __main__
# cleanup[2] removing encodings.latin_1
# cleanup[2] removing io
# destroy io
# cleanup[2] removing abc
# destroy abc
# cleanup[2] removing _weakrefset
# destroy _weakrefset
# cleanup[2] removing encodings.cp437
# destroy zipimport
# destroy zlib
# destroy _signal
# cleanup[3] wiping _frozen_importlib
# destroy _frozen_importlib_external
# cleanup[3] wiping _imp
# cleanup[3] wiping _warnings
# cleanup[3] wiping _thread
# cleanup[3] wiping _weakref
# cleanup[3] wiping _io
# cleanup[3] wiping marshal
# cleanup[3] wiping nt
# cleanup[3] wiping winreg
# cleanup[3] wiping codecs
# cleanup[3] wiping _codecs
# cleanup[3] wiping encodings.aliases
# cleanup[3] wiping encodings.utf_8
# cleanup[3] wiping encodings.latin_1
# cleanup[3] wiping encodings.cp437
# cleanup[3] wiping sys
# cleanup[3] wiping builtins
# destroy _imp
# destroy io
# destroy _warnings
# destroy marshal
# destroy nt
# destroy _thread
# destroy _weakref
# destroy winreg
# destroy _frozen_importlib