Make module map generation conditional
Make module map generation conditional

In PIF generation, we were unconditionally generating a module map, but
this needs to be conditional on whether `clangTarget.moduleMapPath`
exists.

apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager