poetry: Path dependencies with setup.py cannot be installed if setup.py imports `numpy` (or more general, imports any site-packages)

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: Windows 10
• Poetry version: 1.1.4

Issue

I have searched in here and found #2638 is similar but not the same problem as I am facing. I have also tried a possible solution of [build-system].requires and consulted its documentation (as specified in #744) but no success. Here is a snapshot of the error message:

C:\Users\XXX\Desktop\test-module>poetry update -vvv
Updating dependencies
Resolving dependencies...

  PackageInfoError

  Unable to determine package info for path: C:\Users\XXX\Desktop\test-module\lib\my-dev-module

  Fallback egg_info generation failed.

  Command C:\Users\XXX\AppData\Local\Temp\tmpg5w9jw0j\.venv\Scripts\python.exe setup.py egg_info errored with the following return code 1, and output:
  Traceback (most recent call last):
    File "setup.py", line 11, in <module>
      import numpy
  ModuleNotFoundError: No module named 'numpy'

  at ~\.poetry\lib\poetry\inspection\info.py:503 in _pep517_metadata
      499│                     venv.run("python", "setup.py", "egg_info")
      500│                     return cls.from_metadata(path)
      501│                 except EnvCommandError as fbe:
      502│                     raise PackageInfoError(
    → 503│                         path, "Fallback egg_info generation failed.", fbe
      504│                     )
      505│                 finally:
      506│                     os.chdir(cwd.as_posix())
      507│

I will reuse its structure and show how the problem is reproduced.

The project structure is something like

test-module/
  + pyproject.toml
  + testmodule/
      + __init__.py
  + lib/
      + my-dev-module/
          + setup.py
          + mydevmodule/
              + __init__.py

The toml file is

[tool.poetry]
name = "testmodule"
version = "0.1.0"

[tool.poetry.dependencies]
python = "^3.8"
numpy = "^1.19.1"
mydevmodule = {path = "./lib/my-dev-module", develop = true}

[build-system]
requires = ["numpy>=1.19.1,<1.20.0","poetry-core>=1.0.0"]
build-backend = "poetry.core.masonry.api"

And ./lib/my-dev-module/setup.py is like

from distutils.core import setup
import numpy

if __name__ == "__main__":
    setup(
        name="mydevmodule",
        version="0.2",
        description="",
        author="me",
    )

The setup() in setup.py is a placeholder and the script does not get executed to setup(). It erred out at line 2 when importing numpy. In my actual application, module numpy is used to provide its C library for non-Python extension.

My question is: why does it fail at importing numpy? Shouldn’t specifying [build-system].requires fix this? If not, I got that to fix this problem requires a [build-system].requires and a pyproject.toml in mydevmodule but that is not an ideal solution. Is there any other ways to specify the installation time requirements of a path-dependency module? Note that I have put numpy in both the dependencies and build-system requirement of the top-level project testmodule.