False-positive for anonymous functions

Wrapcheck throws an error for the next case:

```go
import "errors"

func Test_AnonFunc(t *testing.T) {
	test := func() error {
		return errors.New("test")
	}
	fmt.Println(test())
}
```

`error returned from external package is unwrapped: sig: func errors.New(text string) error`