go.tools/go.types: init dependencies via method values are not detected

<pre>For instance for:

package p

type T7 int
func (T7) m() int { _ = x7; return 0 }
var x7 = T7(0).m

type T8 int
func (T8) m() int { _ = x8; return 0 }
var x8 = T8(0).m()

no cycles are reported in the declaration of x7 and x8.</pre>
