cmd/dist: reenable TestDeps

It looks like I forgot to reenable this test when I fixed #21522.
Update deps.go and reenable.

Change-Id: I68a45df09b418f48d93d2e7ab1d274e056c192e6
Reviewed-on: https://go-review.googlesource.com/70050
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/dist/deps.go

@@ -334,6 +334,7 @@ var builddeps = map[string][]string{
 		"container/heap",       // cmd/go/internal/work
 		"crypto/sha256",        // cmd/go/internal/work
 		"debug/elf",            // cmd/go/internal/work
+		"encoding/json",        // cmd/go/internal/work
 		"errors",               // cmd/go/internal/work
 		"flag",                 // cmd/go/internal/work
 		"fmt",                  // cmd/go/internal/work

src/cmd/dist/deps_test.go

@@ -15,7 +15,6 @@ import (
 )
 
 func TestDeps(t *testing.T) {
-	t.Skip("broken - golang.org/issue/21522")
 	if testing.Short() && testenv.Builder() == "" {
 		t.Skip("skipping in short mode")
 	}