Support "local overrides" for index packages

[jneem]

Sometimes you want a quick way to hack on one of your dependencies without going to the trouble of making a proper fork. Python supports this use case with "editable" dependencies. Cargo has at least two mechanisms: doubly-specified dependencies with both a path and an index (or git) dep, and "patch" dependencies.

[yannham]

If I understand correctly, you can somehow already do that by editing the dependencies to point to a local path instead of a git repo, right? So is this issue about the ergonomics of the toggle? What would a good solution look like?

[jneem]

Some annoyances with editing the dependencies:

• it involves multiple steps (cloning the dependency repo and then editing the manifest). Editable dependencies with pip are just a single command
• you have to remember to change your manifest back
  • In cargo, the doubly-specified dependencies mechanism automatically gives you a local dependency while working locally, and an index dependency when publishing.
  • In cargo, the paths overrides mechanism keeps the override out of your manifest, so you can have it in a gitignored file.
  • With pip, it's just a flag to pip. I don't think it affects the manifest at all.

I'm not sure that these annoyances are dealbreakers; other solutions have annoyances of their own (like forgetting that you have a .gitignored config file and being surprised by the wrong dependency)