respect variable shadowing and type aliasing

[maratori]

Previously linter didn't understand that variable is shadowed, and didn't complain about following code (with --allow-error-in-defer enabled). Now it's fixed.

func shadowVariable() (err error) {
	defer func() {
		err := errors.New("xxx")
	}()
}

Also now linter checks real type, not only name error (which can also be shadowed).

[firefart]

btw: I made the named error ignore when used in defer now the default and renamed the flag accordingly in v1.0.4. Will update golang-ci once the linter is updated