It sounded like it was solving my issue, but it doesn't seem to change anything w.r.t. DEFINES_MODULE when running swift package generate-xcodeproj (I installed Swift 4.2 and xcode-selected Xcode 10 beta, `swift package --version says "Swift 4.2.0 (swiftpm-14248)").
I am looking into the open source project grpc-swift, so I can be very specific: the target CgRPC doesn't build properly (i.e. CgRPC.framework doesn't contain "Headers" and "Modules"). When manually setting DEFINES_MODULE to "Yes", and adding a Header build phase, the CgRPC target results in a correct framework.
Looking at the target, one can see that the include/ directory contains a header called .h (i.e. grpc.h).
Shouldn't it be detected as an umbrella header, hence fixing the build and enabling DEFINES_MODULE?