Project fails to build under pep517

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If I attempt to build one of the plugins under pep517, it fails with this traceback:

```
root@bf29a9e52d27:/# pip install --use-pep517 'pytest-virtualenv@ git+https://github.com/man-group/pytest-plugins#subdirectory=pytest-virtualenv'
Collecting pytest-virtualenv@ git+https://github.com/man-group/pytest-plugins#subdirectory=pytest-virtualenv
Cloning https://github.com/man-group/pytest-plugins to /tmp/pip-install-n5u0efiw/pytest-virtualenv_59331a8cc84045d8b8b1bacc5cdb9307
Running command git clone -q https://github.com/man-group/pytest-plugins /tmp/pip-install-n5u0efiw/pytest-virtualenv_59331a8cc84045d8b8b1bacc5cdb9307
Installing build dependencies ... done
Getting requirements to build wheel ... error
ERROR: Command errored out with exit status 1:
command: /usr/bin/python3.9 /usr/local/lib/python3.9/dist-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpj2_gha2n
cwd: /tmp/pip-install-n5u0efiw/pytest-virtualenv_59331a8cc84045d8b8b1bacc5cdb9307/pytest-virtualenv
Complete output (18 lines):
Traceback (most recent call last):
File "/usr/local/lib/python3.9/dist-packages/pip/_vendor/pep517/_in_process.py", line 280, in
main()
File "/usr/local/lib/python3.9/dist-packages/pip/_vendor/pep517/_in_process.py", line 263, in main
json_out['return_val'] = hook(**hook_input['kwargs'])
File "/usr/local/lib/python3.9/dist-packages/pip/_vendor/pep517/_in_process.py", line 114, in get_requires_for_build_wheel
return hook(config_settings)
File "/tmp/pip-build-env-g15tkrb_/overlay/lib/python3.9/site-packages/setuptools/build_meta.py", line 162, in get_requires_for_build_wheel
return self._get_build_requires(
File "/tmp/pip-build-env-g15tkrb_/overlay/lib/python3.9/site-packages/setuptools/build_meta.py", line 143, in _get_build_requires
self.run_setup()
File "/tmp/pip-build-env-g15tkrb_/overlay/lib/python3.9/site-packages/setuptools/build_meta.py", line 267, in run_setup
super(_BuildMetaLegacyBackend,
File "/tmp/pip-build-env-g15tkrb_/overlay/lib/python3.9/site-packages/setuptools/build_meta.py", line 158, in run_setup
exec(compile(code, __file__, 'exec'), locals())
File "setup.py", line 6, in
from common_setup import common_setup
ModuleNotFoundError: No module named 'common_setup'
----------------------------------------
WARNING: Discarding git+https://github.com/man-group/pytest-plugins#subdirectory=pytest-virtualenv. Command errored out with exit status 1: /usr/bin/python3.9 /usr/local/lib/python3.9/dist-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpj2_gha2n Check the logs for full command output.
ERROR: Could not find a version that satisfies the requirement pytest-virtualenv (unavailable)
ERROR: No matching distribution found for pytest-virtualenv (unavailable)
WARNING: You are using pip version 20.3.4; however, version 21.3.1 is available.
You should consider upgrading via the '/usr/bin/python3.9 -m pip install --upgrade pip' command.
```

Pep517 uses isolated builds, copying the source tree for installation. Because these plugins require code found outside the source tree, it fails.

Reproduce the isolated PEP 517 build for pytest-virtualenv and inspect its setup.py, especially the import of common_setup. Trace how the plugin source is copied for the build and how common_setup is provided outside that tree. Done means the plugin can be installed successfully with --use-pep517 from the repository URL.

Written by the indexing model from the issue text.