RFC: Swift Package Manager -- Editable Packages Proposal

I am proposing we change the behavior for iterative development of a group of packages. In particular, we will change the default location to which package dependency sources are cloned, the package managers behavior around those sources, and add a new feature for allowing iterative development. These features are tightly interrelated, which is why they are combined into one proposal.

Proposal here:
  swift-evolution/NNNN-swiftpm-package-edit.md at dca2f84ff9dd910c4d8c648da3a3f727ed918274 · ddunbar/swift-evolution · GitHub

Feedback welcome!

- Daniel