Issue Description:

The Swift package manager should have support for a development style where many packages which depend on one another are being concurrently authored, i.e. when a developer wants to work on the trunk (i.e., "master") branch of each repository and have the package manager always use the latest sources for each package.

Currently this style of development requires frequent dummy tagging or manifest rewriting in order for the package manager to pick up the dependencies, which makes the practical workflow very cumbersome.