SA1013: overhaul check

[dominikh]

False positives

We should make sure that we're actually calling io.Seeker.Seek, and not some other method named Seek, to avoid false positives like

type S struct{}

func (S) Seek(int, string)

func fn1() {
	var s S
	s.Seek(io.SeekStart, "")
}

In fact we even look at package-level functions in other packages, not just methods.

Furthermore, we look for the use of io.SeekStart etc, without making sure that this refers to a package level constant or the actual stdlib io package.

False negatives

We only look for the io.Seek* constants, not the deprecated os equivalents.

[dominikh]

We only look for the io.Seek* constants, not the deprecated os equivalents.

This isn't actually a problem, because the os equivalents have types, and can't actually be used in the wrong order (not without explicit conversions, anyway).