Add an option to skip project generation for binary targets (vs throwing an error)

[crleona]

It is sometimes useful to provide both a source and precompiled version of the same product from the same Package.swift file. Unfortunately, we cannot use swift-create-xcframework to build for that case, as any binary targets added to the package will fail project generation with an error, even if they not needed for building the specified product.

This PR offers a simple workaround - the ability to skip adding binary targets if they are not needed for a build.

[bsneed]

Thanks @crleona ! This isn't something I've had to do yet, so thanks for blazing that trail here.