Project with path dependency cannot be installed with `pip install .` (or via requirements file) due to "RequirementParseError: Invalid URL"

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- **OS version and name**: macOS 11.4

- **Poetry version**: 1.1.13, 1.2.0b2.dev0 (f2c365fb6a23e78205681d6aa4111c888e9ee6ab)

- **Link of a [Gist](https://gist.github.com/) with the contents of your pyproject.toml file**: https://github.com/huonw/poetry-path-deps-issue



## Issue



We're working within a monorepo where we have several projects that depend on each other via `path = "..."` relative dependencies. Installing such a project directly with `pip install` fails with `pip._vendor.pkg_resources.RequirementParseError: Invalid URL: ../a`.



Why are we trying to do this? We want to have access to `pip install --target` to install packages to a specific directory, when packaging for an AWS Lambda .zip file. This means we're exporting to a requirements file and installing that.



This is potentially caused by only absolute paths being turned into a URL, in https://github.com/python-poetry/poetry-core/blob/c5f4cda45402c38bb87ab73d0e32de509ba68f41/src/poetry/core/packages/directory_dependency.py#L125



Potentially related issues:



- similar error messages: python-poetry/poetry#3930, python-poetry/poetry#3899, python-poetry/poetry#4709, python-poetry/poetry#4868

- similar set-up: python-poetry/poetry-plugin-export#147

- python-poetry/poetry#936 for the monorepo style with a lot of path deps



### Example



https://github.com/huonw/poetry-path-deps-issue contains a full reproducible example:



```shell

cd /tmp

git clone https://github.com/huonw/poetry-path-deps-issue.git

pip install poetry-path-deps-issue/b

```



The core layout is:



```

.

├── a

│   ├── a

│   │   └── __init__.py

│   └── pyproject.toml

├── b

│   ├── b

│   │   └── __init__.py

│   └── pyproject.toml

└── c

    ├── c

    │   └── __init__.py

    ├── poetry.lock

    └── pyproject.toml

```



Where `b/pyproject.toml` contains a dependency `a = { path = "../a" }` (`c` is left over from an older version of this issue, see below).



The `pip install .` fails with:



```

Processing /poetry-path-deps-issue/b

...

  File "/usr/local/lib/python3.9/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3103, in __init__

    raise RequirementParseError(

[]