[preset] Factor swiftpm and downstream presets
[preset] Factor swiftpm and downstream presets

Factor out the common configuration between the SwiftPM preset and
downstream package presets. This is a win in itself, but it also helps
prepare for making the llbuild jobs build downstream packages in a
future commit.

ahoppen/swift,glessard/swift,benlangmuir/swift,gregomni/swift,allevato/swift,xwu/swift,devincoughlin/swift,parkera/swift,apple/swift,tkremenek/swift,roambotics/swift,parkera/swift,hooman/swift,stephentyrone/swift,airspeedswift/swift,stephentyrone/swift,nathawes/swift,xwu/swift,aschwaighofer/swift,parkera/swift,apple/swift,devincoughlin/swift,atrick/swift,lorentey/swift,hooman/swift,karwa/swift,ahoppen/swift,gregomni/swift,lorentey/swift,karwa/swift,karwa/swift,sschiau/swift,allevato/swift,ahoppen/swift,CodaFi/swift,tkremenek/swift,parkera/swift,stephentyrone/swift,airspeedswift/swift,aschwaighofer/swift,atrick/swift,lorentey/swift,hooman/swift,jmgc/swift,harlanhaskins/swift,gregomni/swift,CodaFi/swift,jckarter/swift,harlanhaskins/swift,aschwaighofer/swift,karwa/swift,stephentyrone/swift,lorentey/swift,atrick/swift,jckarter/swift,hooman/swift,gribozavr/swift,hooman/swift,parkera/swift,benlangmuir/swift,nathawes/swift,parkera/swift,JGiola/swift,karwa/swift,glessard/swift,gregomni/swift,stephentyrone/swift,benlangmuir/swift,parkera/swift,sschiau/swift,JGiola/swift,apple/swift,rudkx/swift,sschiau/swift,gribozavr/swift,sschiau/swift,harlanhaskins/swift,karwa/swift,roambotics/swift,apple/swift,CodaFi/swift,stephentyrone/swift,sschiau/swift,gribozavr/swift,tkremenek/swift,harlanhaskins/swift,nathawes/swift,harlanhaskins/swift,jckarter/swift,devincoughlin/swift,airspeedswift/swift,roambotics/swift,CodaFi/swift,gribozavr/swift,atrick/swift,JGiola/swift,jckarter/swift,glessard/swift,tkremenek/swift,airspeedswift/swift,stephentyrone/swift,CodaFi/swift,gribozavr/swift,jmgc/swift,gregomni/swift,harlanhaskins/swift,benlangmuir/swift,rudkx/swift,devincoughlin/swift,rudkx/swift,allevato/swift,allevato/swift,glessard/swift,atrick/swift,aschwaighofer/swift,airspeedswift/swift,sschiau/swift,xwu/swift,CodaFi/swift,CodaFi/swift,jmgc/swift,rudkx/swift,atrick/swift,rudkx/swift,JGiola/swift,xwu/swift,xwu/swift,roambotics/swift,roambotics/swift,sschiau/swift,jmgc/swift,apple/swift,allevato/swift,rudkx/swift,gribozavr/swift,aschwaighofer/swift,aschwaighofer/swift,lorentey/swift,hooman/swift,ahoppen/swift,tkremenek/swift,tkremenek/swift,xwu/swift,xwu/swift,devincoughlin/swift,JGiola/swift,tkremenek/swift,allevato/swift,nathawes/swift,parkera/swift,benlangmuir/swift,jckarter/swift,glessard/swift,jmgc/swift,JGiola/swift,devincoughlin/swift,gribozavr/swift,gregomni/swift,roambotics/swift,ahoppen/swift,jckarter/swift,ahoppen/swift,apple/swift,lorentey/swift,nathawes/swift,allevato/swift,hooman/swift,harlanhaskins/swift,benlangmuir/swift,glessard/swift,jckarter/swift,airspeedswift/swift,jmgc/swift,karwa/swift,devincoughlin/swift,nathawes/swift,nathawes/swift,jmgc/swift,gribozavr/swift,aschwaighofer/swift,sschiau/swift,airspeedswift/swift,devincoughlin/swift,karwa/swift,lorentey/swift,lorentey/swift