Support extras to override a dependency

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.

Feature Request

I have a project that depends on a public pypi pacakge - however, for some installs, I would like them to be able to override the package themselves as an extra without having to override the dependency in their setup

Current functionality

[tool.poetry]
name = "my-awesome-package"

[tool.poetry.dependencies]
dependency = "^1.0"

User runs

pip install my-awesome-package git+https://github.com/forked/dependency

And deals with any version conflicts since it could be out of sync.

Desired functionality

[tool.poetry]
name = "my-awesome-package"

[tool.poetry.dependencies]
dependency = [ {version = "^1.0"}, { git = "https://github.com/forked/dependency", optional=true }

[tool.poetry.extras]
custom = ["dependency"]

User runs

pip install my-awesome-package[custom]

No files, tests, or entry points are named in the issue. Begin by tracing Poetry's dependency and extras handling, then define how an optional alternate source would be represented and validated; done means installing [custom] selects the override without changing the default dependency.