var errMissing = errors.New("cannot find package")

// loadFromRoots attempts to load the build graph needed to process a set of
// root packages and their dependencies.
//
// The set of root packages is returned by the params.listRoots function, and
// expanded to the full set of packages by tracing imports (and possibly tests)
// as needed.
func loadFromRoots(ctx context.Context, params loaderParams) *loader {
	ld := &loader{