// loadImports populates w with information about the packages in the standard
// library and the packages they themselves import in w's build context.
//
// The source import path and expanded import path are identical except for vendored packages.
// For example, on return:
//
//	w.importMap["math"] = "math"
//	w.importDir["math"] = "<goroot>/src/math"
//
//	w.importMap["golang.org/x/net/route"] = "vendor/golang.org/x/net/route"