dependencies installed from VCS leads to a failure

[tyong920]

Describe the bug
The build failed for a project which contains dependencies installed from VCS.
To Reproduce
Steps to reproduce the behavior:

1. Go to 'tests/data/lock'
2. Run pdm sync
3. Add "pdm-build-locked" into requires = ["pdm-backend", "pdm-build-locked"]
4. Run pdm build succeed
5. Add a VCS style dependence failed, e.g., pdm add git+https://github.com/psf/[email protected]
6. See error pdm.backend._vendor.pyproject_metadata.ConfigurationError: Field "project.optional-dependencies.locked" contains an invalid PEP 508 requirement string "black==24.4.2 @ git+https://github.com/psf/black.git@3702ba224ecffbcec30af640c149f231d90aebdb" ("Expected end or semicolon (after version specifier) black==24.4.2 @ git+https://github.com/psf/black.git@3702ba224ecffbcec30af640c149f231d90aebdb ~~~~~~~~~^")

Desired behavior
pdm-build-locked shall work well with VCS dependencies.