Created[i] contains the initial package whose ASTs or filenames were supplied by Config.CreatePkgs[i], followed by the external test package, if any, of each package in Config.ImportPkgs ordered by ImportPath.

NOTE: these files must not import "C". Cgo preprocessing is only performed on imported packages, not ad hoc packages.

TODO(adonovan): we need to copy and adapt the logic of goFilesPackage (from $GOROOT/src/cmd/go/build.go) and make Config.Import and Config.Create methods return the same kind of entity, essentially a build.Package. Perhaps we can even reuse that type directly.