`skip init` should create `Package.swift` in a subdirectory

Apropos #448, the Xcode workspace structure for a generated project is (still) extremely strange.

skip init creates an Xcode project .xcodeproj in the ./Darwin folder, and creates a ./Package.swift file in the generated root folder.

To make the generated library visible in the project, the root ./ folder is added to the .xcodeproj by reference, and the library name e.g. ModuleName is added as a dependency of the app project. A few file references are tossed in to the project, too, including README.md, Skip.env, and Localizable.xcstrings.

I think the normal way to have an app project that references a Swift library is to have the library appear in a ModuleName subdirectory next to the Darwin directory. Instead of:

Project.xcworkspace
Darwin/ModuleName.xcodeproj
Package.swift
Sources/ModuleName/ModuleNameApp.swift

It would look like this:

Project.xcworkspace
Darwin/ModuleName.xcodeproj
ModuleName/Package.swift
ModuleName/Sources/ModuleNameApp.swift

This would make the project structure much less confusing, IMO.

Start at the implementation of skip init and inspect how it generates Package.swift, Darwin/ModuleName.xcodeproj, and Sources/ModuleName/ModuleNameApp.swift. Compare the generated output with the proposed layout, then verify that the package is created under ModuleName beside Darwin and that the generated project still references it correctly.