fix: issue 1797

[baszalmstra]

This PR contains two changes.

• When converting from a pyproject manifest to an internal manifest, errors can occur, these are now handled properly.
• I changed add_pypi_dependency to add_pep508_dependency when the function accepts a pep508 requirement.
• Parsing of git urls with @ works properly now but might still fail if not pointing to a revision (but a branch instead). The error message does now reflect this shortcoming.

Fixes #1797 as the code no longer panics but does still result in an error.