Enable adding local Swift packages to the project root

[hiltonc]

This enables adding local Swift packages to the project root. This can be done in one of two ways:

1. Use a "" group for a package:

packages:
  MyPackage:
    path: MyPackage
    group: ""

2. Use "" for the localPackagesGroup option:

options:
  localPackagesGroup: ""

Resolves #1209

[yonaskolb]

Thank you for this @hiltonc. Could you resolve the conflicts and make sure it still works after https://github.com/yonaskolb/XcodeGen/pull/1417 was merged

[OmranK]

Would this solution allow you to have Xcode project with local packages generated like this?

[hiltonc]

@OmranK yes, it should enable you to do that

[hiltonc]

@yonaskolb I've resolved conflicts, tests pass, and I manually verified both configuration options.