interp: handle alias of an alias

When dealing with an alias of an alias, the actual underlying type of the source alias should be used.

_test/alias4.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"fmt"
+	"net/http"
+)
+
+type A http.Header
+
+func (a A) Test1() {
+	fmt.Println("test1")
+}
+
+type B A
+
+func (b B) Test2() {
+	fmt.Println("test2")
+}
+
+func main() {
+	b := B{}
+
+	b.Test2()
+}
+
+// Output:
+// test2