Fix: Detect erroneous multiplication assignments (x *= y)

[coxley]

Summary

This addresses a gap where multiplication assignments were not checked:

package main

import "time"

func main() {
	x := time.Second
	x = x * time.Hour  // durationcheck: Multiplication of durations
	x *= time.Hour     // nothing
}

I recommend this linter occasionally, but zimpenfish pointed out it missed this case in some of their code: https://lobste.rs/c/7pbivg

PS: I updated go.mod to 1.16. We have a dependency on golang.org/x/tools@v0.6.0, which uses io.ReadAll. The dependency itself declares go 1.18 // tagx:compat 1.16, but figured I'd avoid upping the requirement more than necessary.

Test Plan

> docker run -it -v=$(pwd):/app --workdir=/app --rm golang:1.16 sh -c 'go mod tidy && go test'
go: downloading golang.org/x/tools v0.6.0
go: downloading golang.org/x/sys v0.5.0
go: downloading golang.org/x/mod v0.8.0
go: downloading golang.org/x/sync v0.1.0
PASS
ok      github.com/charithe/durationcheck       0.774s

Using Docker because the tests are broken with Go binaries >= 1.22, but that's not our fault: golang/go#75918 (comment)

[charithe]

Thank you for the fix and the really helpful PR description. I haven't touched this project for a while and didn't notice that issue with Go 1.22 and above. I wonder how the golangci-lint folks manage to build it.