core: intree hooks fails to detect compatibility requirements

[abn]

$ docker run --rm -i python:2.7 bash <<EOF
python -m pip install poetry-core@git+https://github.com/python-poetry/poetry-core.git@master
EOF
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. A future version of pip will drop support for Python 2.7. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support
Collecting poetry-core@ git+https://github.com/python-poetry/poetry-core.git@master
  Cloning https://github.com/python-poetry/poetry-core.git (to revision master) to /tmp/pip-install-0UVY_a/poetry-core
  Running command git clone -q https://github.com/python-poetry/poetry-core.git /tmp/pip-install-0UVY_a/poetry-core
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'error'
  ERROR: Command errored out with exit status 1:
   command: /usr/local/bin/python /usr/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpoeMUL3
       cwd: /tmp/pip-install-0UVY_a/poetry-core
  Complete output (18 lines):
  Traceback (most recent call last):
    File "/usr/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 257, in <module>
      main()
    File "/usr/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 240, in main
      json_out['return_val'] = hook(**hook_input['kwargs'])
    File "/usr/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 91, in get_requires_for_build_wheel
      return hook(config_settings)
    File "/tmp/pip-build-env-F0iBuD/overlay/lib/python2.7/site-packages/intreehooks.py", line 53, in get_requires_for_build_wheel
      return self._backend.get_requires_for_build_sdist(config_settings)
    File "/tmp/pip-build-env-F0iBuD/overlay/lib/python2.7/site-packages/intreehooks.py", line 38, in _backend
      obj = self._module_from_dir(modname)
    File "/tmp/pip-build-env-F0iBuD/overlay/lib/python2.7/site-packages/intreehooks.py", line 25, in _module_from_dir
      mod = importlib.import_module(modname)
    File "/usr/local/lib/python2.7/importlib/__init__.py", line 37, in import_module
      __import__(name)
    File "/tmp/pip-install-0UVY_a/poetry-core/poetry/core/__init__.py", line 8, in <module>
      from pathlib2 import Path
  ImportError: No module named pathlib2
  ----------------------------------------
ERROR: Command errored out with exit status 1: /usr/local/bin/python /usr/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpoeMUL3 Check the logs for full command output.
WARNING: You are using pip version 20.0.2; however, version 20.2.3 is available.
You should consider upgrading via the '/usr/local/bin/python -m pip install --upgrade pip' command.

The issue seems to that when installing from vcs, the build backend does not correctly detect the compatibility requirements specified by the package.

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