Add Swift package manager support (#1890)
Add Swift package manager support (#1890)

Previously the only Swift maker was just for compiling a single file
with no arguments, now if there is a Package.swift file in the current
directory, we will use `swift build` instead.
nerdrew/neomake,codeinabox/neomake,nerdrew/neomake,neomake/neomake,neomake/neomake