[swift-build-dev] More layering changes...

[Daniel Dunbar]

The layering refactoring is now done, PRs are safe from gratuitous conflicting once again!

I have added a couple README files, e.g.:
  https://github.com/apple/swift-package-manager/blob/master/Sources/PackageModel/README.md
to documentation the layering intent for each module, and our own `Package.swift` has an overview of the structure:
  https://github.com/apple/swift-package-manager/blob/master/Package.swift

> On May 18, 2016, at 9:01 AM, Anders Bertelrud <anders at apple.com> wrote:
> 
> On 2016-05-17, at 17.26, Daniel Dunbar via swift-build-dev <swift-build-dev at swift.org> wrote:
> 
>> PackageModel (renamed from PackageType):
>> - This defines the basic model objects for Packages. The rename is to try and convey the intent more clearly. This module is intended to define "pure" model objects.
> 
> Just to clarify what we mean by “pure” here — there should be no conventions or other magic applied at this level, and no intermixed resolution policies etc.  That’s added by higher levels.  Also, at this level there are no connection between packages — that happens in PackageGraph.

Indeed. I stole your pure terminology for the README above.

 - Daniel

> 
> Anders
>