fix(getter): race condition when SetClient is called

[srevinsaju]

This is a non-breaking change. This fixes an issue when Clients are created and called concurrently.
Clients called SetClient on each of the Getters. When no getters were provided, it modified the clients of the default getters defined in the Getters map.
Since the map was mutated from multiple goroutines, it creates a race condition 'concurrent map writes'. The issue is mitigated by creating a new copy of Getters using DefaultGetters() function for every new client created. SetClient now modifies its own copy of the Getter, not the default Getters.

To reproduce the issue:

mkdir hello
touch hello/world

// main.go
package main

import "github.com/hashicorp/go-getter"
import "context"
import "os"
import "sync"
import "path/filepath"

func main() {
    var wg sync.WaitGroup
    cwd, err := os.Getwd()
    if err != nil {
        panic(err)
    }
    keys := []string{"a", "b", "c"}

    for _, key := range keys {
        wg.Add(1)
        go func(key string) {
            get := &getter.Client{
                Ctx: context.Background(),
                Src: "./hello",
                Dst: filepath.Join("dest", key),
                Pwd: cwd,
                Dir: true,
            }
      err := get.Get()
            if err != nil {
                panic(err)
            }
            wg.Done()
        }(key)
    }
    wg.Wait()
}

go run -race .

[hashicorp-cla]

All committers have signed the CLA.

[CreatorHead]

Thanks for the fix, the race is real. One concern: this changes behavior when Client.Getters is nil. Previously we used the package-level Getters map, which many consumers may customize (e.g. adding schemes). With c.Getters = DefaultGetters() we would ignore those customizations. Can we instead copy the global Getters map into a per-client map (so we avoid shared mutation, but still honor user overrides)? Also, could you add a small test that reproduces the race with -race and verifies the fix?