Use new golang.org/x/... import paths

See http://golang.org/s/go14subrepo

Fix the following error with go 1.3.3:
    ./structcheck.go:166: cannot use imp.Import (type func(map[string]*"golang.org/x/tools/go/types".Package, string) (*"golang.org/x/tools/go/types".Package, error)) as type "code.google.com/p/go.tools/go/types".Importer in field value