go/types, types2: error messages don't distinguish between different types with the same names

[griesemer]

package p

type T int

func _() {
	type GlobalT = T
	type T int
	var _ T = GlobalT(0)
}

produces:

x.go:8:12: incompatible type: cannot use GlobalT(0) (constant 0 of type T) as T value

The error message reports the same type (T) for the variable and the constant, which is confusing.

Maybe we can detect cases like these and provide additional information, such as where the types are declared.

Not urgent as this is a rare scenario.