Bias towards local directories for bare editable requirements

[charliermarsh]

Summary

Given install -e dagster, we need to assume that the user meant install -e ./dagster, even though install dagster should not be treated as install ./dagster. I suspect pip will change this in the future (since pip install dagster does not meant pip install ./dagster) but for now it's what users expect.

Closes #3994.

[charliermarsh]

Needs tests.

[codspeed-hq]

CodSpeed Performance Report

Merging #3995 will not alter performance

_{Comparing charlie/pref (787920a) with main (420333a)}

Summary

✅ 13 untouched benchmarks

[konstin]

Is there a pip issue to subscribe to?

[charliermarsh]

I only see pypa/pip#1289 (comment) where they mention that lack of PEP 508 support for editables is an implementation issue, not a design issue. But if they change editables to support PEP 508, I think they'll have the same behavior as us (prior to this PR), unless they also special-case it?

[charliermarsh]

I asked in pip here: pypa/pip#12745