Issue when installing existing addon from same repo from a PR

[sbidoul]

Say you have a repo with addons A and B.

Say you have a PR with changes to B that depend on a changes in an unmerged PR to A . In that PR, test-requirements.txt has a git reference to A.

In that case, it is unclear if the PR to A will have precendence over A found in the addons path (it is deterministic but fragile and not guaranteed to not change in the future).

When using editable installation mode (#62) there will likely be a conflict between the PR to A and -e A that pip will not be able to resolve.