Expand environment variables in URLs (#640)
## Summary
This PR enables users to express relative dependencies via environment
variables. Like pip, PDM, Hatch, Rye, and others, we now allow users to
express dependencies like:
```text
flask @ file://${PROJECT_ROOT}/flask-3.0.0-py3-none-any.whl
```
In the compiled requirements file, we'll also preserve the unexpanded
environment variable.
Closes https://github.com/astral-sh/puffin/issues/592.
