This was originally a separate library since it was copied directly
from the legacy SwiftPM library. However it's since been changed a
bunch, and ought to be able leverage SwiftXcodeGen utilities such as
RelativePath. Fold it into SwiftXcodeGen.
I considered splitting out the utility code from SwiftXcodeGen into
a new library, but unfortunately that currently regresses performance
(even with max CMO). The module organization doesn't really currently
matter since we don't expose any products.
This is a tool specifically designed to generate
Xcode projects for the Swift repo (as well as a
couple of adjacent repos such as LLVM and Clang).
It aims to provide a much more user-friendly experience
than the CMake Xcode generation (`build-script --xcode`).
