Allow relative paths and environment variables in all editable representations

[charliermarsh]

Summary

I don't know if this is actually a good change, but it tries to make the editable install experience more consistent. Specifically, we now support...

# Use a relative path with a `file://` prefix.
# Prior to this PR, we supported `file:../foo`, but not `file://../foo`, which felt inconsistent.
-e file://../foo

# Use environment variables with paths, not just URLs.
# Prior to this PR, we supported `file://${PROJECT_ROOT}/../foo`, but not the below.
-e ${PROJECT_ROOT}/../foo

Importantly, -e file://../foo is actually not supported by pip... -e file:../foo is supported though. We support both, as of this PR. Open to feedback.

[charliermarsh]

This is now just a string.

[charliermarsh]

This is janky.