Option to exclude `site-packages` from python path

PEP 582, as it is today only adds __pypackages__ to the list of discovery paths. This might lead to the usage of globally installed packages by mistake.

I think it would be very useful if it were possible to remove global python paths on a per project basis. Maybe a flag on the pyproject.toml file.

Assuming a project in /Users/foo/projects/bar on macOS using python3.8 installed using official installers:

• Current path list

  ['',
   '/Users/foo/projects/bar/pdm/__pypackages__/3.8/lib',
   '/Users/foo/projects/bar/pdm',
   '/Library/Frameworks/Python.framework/Versions/3.8/lib/python38.zip',
   '/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8',
   '/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/lib-dynload',
   '/Users/foo/Library/Python/3.8/lib/python/site-packages',
   '/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages'
  ]
  

• Suggested path list

  ['',
   '/Users/foo/projects/bar/pdm/__pypackages__/3.8/lib',
   '/Users/foo/projects/bar/pdm',
   '/Library/Frameworks/Python.framework/Versions/3.8/lib/python38.zip',
   '/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8',
   '/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/lib-dynload',
  ]
  

Is this something that you are open to?

[frostming]

@mgzenitech it turns out to be a corner case that the running env overrides the preset ones. It is handled in #1211

[frostming]

Still waiting for the CI to pass(something is going wrong with GitHub actions), the next release should come out in the next 2 days after the PR is merged.