zope.event is not packaged properly when installed together with zope.interface

Nobody has claimed this yet.

When I package a python package which has a dependency on both `zope.event` and `zope.interface`, `zope.event` is not importable. Example case:

```
[15:05:06] λ shiv -o ~/bin/zope_event zope.event zope.interface --upgrade
Collecting zope.event
Using cached zope.event-4.5.0-py2.py3-none-any.whl (6.8 kB)
Collecting zope.interface
Using cached zope.interface-5.4.0-cp39-cp39-manylinux2010_x86_64.whl (255 kB)
Collecting setuptools
Using cached setuptools-57.4.0-py3-none-any.whl (819 kB)
Installing collected packages: setuptools, zope.interface, zope.event
Successfully installed setuptools-57.4.0 zope.event-4.5.0 zope.interface-5.4.0

[15:05:18] λ echo "import zope.event" | SHIV_INTERPRETER=1 python -S ~/bin/zope_event
Python 3.9.6 (default, Jul 16 2021, 00:00:00)
[GCC 11.1.1 20210531 (Red Hat 11.1.1-3)] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> Traceback (most recent call last):
File "", line 1, in
ModuleNotFoundError: No module named 'zope.event'
>>>
now exiting InteractiveConsole...
```

(my original case is packaging https://github.com/Fizzadar/pyinfra/ with `shiv -c pyinfra -o ~/bin/pyinfra pyinfra`, which has this dependency)

Start by reproducing the documented shiv command with zope.event and zope.interface, then run the shown `import zope.event` check under Python 3.9. Investigate how shiv packages these dependencies together; done means the generated executable can import zope.event successfully.

Written by the indexing model from the issue text.