Issue Description:

Hi, I am working on this project right now: Perfect SPNEGO which is using a module map like this:

module cspnego {
  header "cspnego.h"
  link "gssapi_krb5"
  link "cspnego"
  export *
}

Running `swift build` will work both on Mac & Linux; However, the Xcode project generated by `swift package generate-xcodeproj` needs to add a GSS framework manually otherwise it won't build.

I noticed that such a framework requires `ld -framework GSS` other than `ld -lGSS`, so what should I do to the module map to tell swift package manager add this framework?

Thank you very much!