pip install with relative paths in pyproject.toml doesn't seem to work

How do I specify relative paths that pip can use in pyproject.toml?
poetry successfully installs them, however pip fails with the following error:

File "/tmp/pip-build-env-ck09aiax/overlay/lib/python3.7/site-packages/poetry/core/packages/directory_dependency.py", line 36, in __init__
        raise ValueError("Directory {} does not exist".format(self._path))
    ValueError: Directory ../lib_foo does not exist

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Example

I have the following repository structure

.
├── lib_foo
│   ├── lib_foo
│   │   └── __init__.py
│   └── pyproject.toml
└── main_app
    ├── main_app
    │   └── __init__.py
    └── pyproject.toml

main_app is dependent upon lib_foo, its dependecies looks like this:
./main_app/pyproject.toml

[tool.poetry.dependencies]
python = "^3.7"
lib-foo = { path = "../lib_foo" }  # also tried  path = "../lib_foo/"

when doing:

./main_app$ poetry install  # runs successfully 
./main_app$ python -m pip install ../lib_foo/ # runs successfully
./main_app$ python -m pip install .  # fails with the error mentioned above 
./main_app$ python -m pip --version
pip 20.1.1 from ./main_app/my_venv/lib/python3.7/site-packages/pip (python 3.7)

---

I am using poetry when developing a project however when deploying them I want to only use pip

[uranusjr]

You should not be able to build the package, since it is not allowed to have dependencies specified as relative paths in package metadata. PEP 508 only allows version specifiers and URL (which only allows absolute paths even if you use file://). Poetry should fix this.

[uranusjr]

Consequence of #7555. May be resolved with #9091.