Add explicit error message for URLs without package names (#669)
`pip` supports installing packages without names (e.g.,
`git+https://github.com/pallets/flask.git`), but it doesn't adhere to
the PEP grammar, and we don't yet support it (and may never) (#313).
This PR adds a dedicated error path for such cases, to ensure that we
can give meaningful feedback to the user:
```
error: Couldn't parse requirement in requirements.in position 0 to 18
Caused by: URL requirement is missing a package name; expected: `package_name @ https://google.com`
https://google.com
^^^^^^^^^^^^^^^^^^
```
Closes https://github.com/astral-sh/puffin/issues/650.
