util/syspolicy/*: move syspolicy keys to new const leaf "pkey" package

This is step 1 of ~3, breaking up #14720 into reviewable chunks, with
the aim to make syspolicy be a build-time configurable feature.

In this first (very noisy) step, all the syspolicy string key
constants move to a new constant-only (code-free) package. This will
make future steps more reviewable, without this movement noise.

There are no code or behavior changes here.

The future steps of this series can be seen in #14720: removing global
funcs from syspolicy resolution and using an interface that's plumbed
around instead. Then adding build tags.

Updates #12614

Change-Id: If73bf2c28b9c9b1a408fe868b0b6a25b03eeabd1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/syspolicy/source/env_policy_store_test.go

@@ -11,13 +11,14 @@ import (
 	"strconv"
 	"testing"
 
+	"tailscale.com/util/syspolicy/pkey"
 	"tailscale.com/util/syspolicy/setting"
 )
 
 func TestKeyToEnvVarName(t *testing.T) {
 	tests := []struct {
 		name    string
-		key     setting.Key
+		key     pkey.Key
 		want    string // suffix after "TS_DEBUGSYSPOLICY_"
 		wantErr error
 	}{
@@ -166,7 +167,7 @@ func TestEnvPolicyStore(t *testing.T) {
 	}
 	tests := []struct {
 		name    string
-		key     setting.Key
+		key     pkey.Key
 		lookup  func(string) (string, bool)
 		want    any
 		wantErr error