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/internal/metrics/metrics.go

@@ -17,6 +17,7 @@ import (
 	"tailscale.com/util/slicesx"
 	"tailscale.com/util/syspolicy/internal"
 	"tailscale.com/util/syspolicy/internal/loggerx"
+	"tailscale.com/util/syspolicy/pkey"
 	"tailscale.com/util/syspolicy/setting"
 	"tailscale.com/util/testenv"
 )
@@ -209,7 +210,7 @@ func scopeMetrics(origin *setting.Origin) *policyScopeMetrics {
 
 var (
 	settingMetricsMu  sync.RWMutex
-	settingMetricsMap map[setting.Key]*settingMetrics
+	settingMetricsMap map[pkey.Key]*settingMetrics
 )
 
 func settingMetricsFor(setting *setting.Definition) *settingMetrics {
@@ -283,8 +284,8 @@ func SetHooksForTest(tb testenv.TB, addMetric, setMetric metricFn) {
 	lazyUserMetrics.SetForTest(tb, newScopeMetrics(setting.UserSetting), nil)
 }
 
-func newSettingMetric(key setting.Key, scope setting.Scope, suffix string, typ clientmetric.Type) metric {
-	name := strings.ReplaceAll(string(key), string(setting.KeyPathSeparator), "_")
+func newSettingMetric(key pkey.Key, scope setting.Scope, suffix string, typ clientmetric.Type) metric {
+	name := strings.ReplaceAll(string(key), string(pkey.KeyPathSeparator), "_")
 	name = strings.ReplaceAll(name, ".", "_") // dots are not allowed in metric names
 	return newMetric([]string{name, metricScopeName(scope), suffix}, typ)
 }

util/syspolicy/internal/metrics/metrics_test.go

@@ -10,13 +10,14 @@ import (
 	"tailscale.com/types/lazy"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/syspolicy/internal"
+	"tailscale.com/util/syspolicy/pkey"
 	"tailscale.com/util/syspolicy/setting"
 )
 
 func TestSettingMetricNames(t *testing.T) {
 	tests := []struct {
 		name           string
-		key            setting.Key
+		key            pkey.Key
 		scope          setting.Scope
 		suffix         string
 		typ            clientmetric.Type