SE-0292 revision 2 (#1301)

changes: 
* Change package identity format to scope.package-name
  This change was made at the request of the Swift core team ahead of second review
* Mention SE-0305 and possibility of distributing artifact archives
* Rename section 'Module name collision resolution' to 'Package name collision resolution'
* .swiftpm/config is now a directory
   This change was made at the request of the Swift core team ahead of second review
* Add HTTP client implementation details under 'Dependency graph resolution'
   This change was made at the request of the Swift core team ahead of second review
* A package's *name* uniquely identifies a package in a scope
   This change was made at the request of the Swift core team ahead of second review

Author: mattt