Support PEP 508 specific extensions

[davidbrochart]

Problem description

See https://github.com/prefix-dev/pixi/issues/1186#issuecomment-2058601959.

I cannot use pixi with jupyter-collaboration:

git clone https://github.com/jupyterlab/jupyter-collaboration.git
cd jupyter-collaboration
pixi init

  × failed to parse project manifest
    ╭─[pyproject.toml:11:16]
 10 │     requires-python = ">=3.8"
 11 │ ╭─▶ dependencies = [
 12 │ │       # jupyter-server extensions
 13 │ │       "jupyter-server-ydoc @ {root:uri}/projects/jupyter-server-ydoc",
 14 │ │       # jupyterlab/notebook frontend extensions
 15 │ │       "jupyter-collaboration-ui @ {root:uri}/projects/jupyter-collaboration-ui",
 16 │ │       "jupyter-docprovider @ {root:uri}/projects/jupyter-docprovider",
 17 │ │       # the metapackage
 18 │ │       "jupyter-collaboration @ {root:uri}/projects/jupyter-collaboration",
 19 │ ├─▶ ]
    · ╰──┤ relative path without a working directory: {root:uri}/projects/jupyter-server-ydoc
    ·    │ jupyter-server-ydoc @ {root:uri}/projects/jupyter-server-ydoc
    ·    │                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 20 │     dynamic = ["version"]
    ╰────