gc: record full package paths in runtime type data

detect compilation of special package runtime with
compiler flag instead of package name.

R=ken2
CC=golang-dev
https://golang.org/cl/193080

src/pkg/reflect/all_test.go

@@ -5,6 +5,7 @@
 package reflect_test
 
 import (
+	"container/vector"
 	"io"
 	"os"
 	. "reflect"
@@ -1188,3 +1189,9 @@ func TestFieldByName(t *testing.T) {
 		}
 	}
 }
+
+func TestImportPath(t *testing.T) {
+	if path := Typeof(vector.Vector{}).PkgPath(); path != "container/vector" {
+		t.Errorf("Typeof(vector.Vector{}).PkgPath() = %q, want \"container/vector\"", path)
+	}
+}