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/policy_keys_test.go

@@ -14,14 +14,19 @@ import (
 	"strconv"
 	"testing"
 
+	"tailscale.com/util/syspolicy/pkey"
 	"tailscale.com/util/syspolicy/setting"
 )
 
 func TestKnownKeysRegistered(t *testing.T) {
-	keyConsts, err := listStringConsts[Key]("policy_keys.go")
+	const file = "pkey/pkey.go"
+	keyConsts, err := listStringConsts[pkey.Key](file)
 	if err != nil {
 		t.Fatalf("listStringConsts failed: %v", err)
 	}
+	if len(keyConsts) == 0 {
+		t.Fatalf("no key constants found in %s", file)
+	}
 
 	m, err := setting.DefinitionMapOf(implicitDefinitions)
 	if err != nil {