Rename "develop" option to "editable"

[abn]

We should rename the dependency option develop to editable to align with what it actually is and to avoid confusion with a
"development dependency".

- foo = { path = "./path/to/foo", develop = true }
+ foo = { path = "./path/to/foo", editable = true }

Additionally we should also introduce this as a commandline option to poetry add (eg: poetry add --editable x).

Changes Required

• Update specification schema in poetry-core.
• Update dependency instances to reference editable and fallback to develop if not present.
• Update references to Dependency.develop in both poetry-core and poetry.
• Add deprecation warning when accessing Dependency.develop.
• Add poetry check warning to inform users about the change.
• Add --editable option directory and vcs dependencies to poetry add command. (This can be an issue on its own.)

[0scarB]

@python-poetry/triage I wouldn't mind working on this, given I've gained a bit of knowledge working on #3220. I could also fix the issue of the bad error message mentioned in #3220

[finswimmer]

Feel free to start working on the other points and ping us if we should give feedback. 👍

[odravison]

I came here because I've been confused about this name.
Would be a good thing do that change

[dimbleby]

this issue has been dormant for three years. I think it's safe to say that if you care to see this happen: you'll need to make this happen.