Feature request: Support for private repositories

[stevapple]

Currently when a subpackage depends on a package from private repository, you need to specify both the package dependency and the source under [[tool.poetry.source]] **in the root pyproject.toml. We hope there could be some support for completely hide the settings in the sub package pyproject.toml. Alternatively, it's also reasonable to specify the dependency in a sub package and the registry in the root, without having to repeat the dependency item.