use empty package-dir in pyproject.toml

[richrines1]

fixes: #868

this appears to be a known issue stemming from the use of import hooks to comply with pep 660 - see for example python/mypy#13392 and pypa/setuptools#3518. What's happening is that for editable installs setuptools creates files like .../site-packages/__editable__.general_superstaq-0.5.5.pth which are supposed to point python to the source directory. Previously these would just be text files containing a single path, e.g.

/<...>/client-superstaq/general-superstaq

but after switching to pyproject.toml it becomes an executable hook, e.g.

import __editable___general_superstaq_0_5_5_finder; __editable___general_superstaq_0_5_5_finder.install()

where __editable___general_superstaq_0_5_5_finder.py is another file saved in the site-packages directory. this breaks static analysis tools because they won't execute the required code

afaict this pr seems to be the cleanest workaround - after these changes setuptools seems to generate the old-style .pth files (i.e. text files containing paths to the source directories), and mypy behaves as expected (at least for me)