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/setting/raw_item.go

@@ -11,6 +11,7 @@ import (
 	"github.com/go-json-experiment/json/jsontext"
 	"tailscale.com/types/opt"
 	"tailscale.com/types/structs"
+	"tailscale.com/util/syspolicy/pkey"
 )
 
 // RawItem contains a raw policy setting value as read from a policy store, or an
@@ -169,4 +170,4 @@ func (v *RawValue) UnmarshalJSON(b []byte) error {
 }
 
 // RawValues is a map of keyed setting values that can be read from a JSON.
-type RawValues map[Key]RawValue
+type RawValues map[pkey.Key]RawValue