a way to import modules from the project root in pdm_build.py

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original issue: #245

Is your feature/enhancement proposal related to a problem? Please describe.

i want my pdm_build.py to be able to import from other modules in my project, but this doesn't seem to be possible.

# ./foo.py
foo = 1

# ./pdm_build.py
from foo import foo

but when running pdm install, the foo module seems inaccessible from pdm_build.py:

ModuleNotFoundError: No module named 'foo'

Describe the solution you'd like

a way to make other modules in my project accessible from pdm_build.py