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.
