rules/sdk: implement pass to only permit key iteration over maps, not value

[odeke-em]

This pass exists to curtail non-determinism in the cosmos-sdk
which stemmed from iterating over maps during upgrades and that
caused a chaotic debug for weeks. With this change, we'll now
enforce and report failed iterations, with the rule being that
a map in a range should involve ONLY one of these 2 operations:

• for k := range m { delete(m, k) } for fast map clearing
• for k := range m { keys = append(keys, k) } to retrieve keys & sort

thus we shall get this report:

[gosec] 2021/11/09 03:18:57 Rule error: *sdk.mapRanging => the value in the range statement should be nil: want: for key := range m (main.go:19)
[gosec] 2021/11/09 03:18:57 Rule error: *sdk.mapRanging => the value in the range statement should be nil: want: for key := range m (main.go:27)

from the code below:

package main

func main() {
	m := map[string]int{
		"a": 0,
		"b": 1,
		"c": 2,
		"d": 3,
	}

	makeMap := func() map[string]string { return nil }

	keys := make([]string, 0, len(m))
	for k := range m {
		keys = append(keys, k)
	}

	values := make([]int, 0, len(m))
	for _, value := range m {
		values = append(values, value)
	}

	type kv struct {
		k, v interface{}
	}
	kvL := make([]*kv, 0, len(m))
	for k, v := range m {
		kvL = append(kvL, &kv{k, v})
	}

	for k := range m {
		delete(m, k)
	}

	for k := range makeMap() {
		delete(m, k)
	}

	for k := range do() {
		delete(m, k)
	}
}

func do() map[string]string { return nil }

Updates cosmos/cosmos-sdk#10190