control/ts2021: rename from internal/noiseconn in prep for controlclient split

A following change will split out the controlclient.NoiseClient type
out, away from the rest of the controlclient package which is
relatively dependency heavy.

A question was where to move it, and whether to make a new (a fifth!)
package in the ts2021 dependency chain.

@creachadair and I brainstormed and decided to merge
internal/noiseconn and controlclient.NoiseClient into one package,
with names ts2021.Conn and ts2021.Client.

For ease of reviewing the subsequent PR, this is the first step that
just renames the internal/noiseconn package to control/ts2021.

Updates #17305

Change-Id: Ib5ea162dc1d336c1d805bdd9548d1702dd6e1468
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/noise_test.go

@@ -15,7 +15,7 @@ import (
 
 	"golang.org/x/net/http2"
 	"tailscale.com/control/controlhttp/controlhttpserver"
-	"tailscale.com/internal/noiseconn"
+	"tailscale.com/control/ts2021"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tailcfg"
@@ -310,7 +310,7 @@ func (up *Upgrader) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		// https://httpwg.org/specs/rfc7540.html#rfc.section.4.1 (Especially not
 		// an HTTP/2 settings frame, which isn't of type 'T')
 		var notH2Frame [5]byte
-		copy(notH2Frame[:], noiseconn.EarlyPayloadMagic)
+		copy(notH2Frame[:], ts2021.EarlyPayloadMagic)
 		var lenBuf [4]byte
 		binary.BigEndian.PutUint32(lenBuf[:], uint32(len(earlyJSON)))
 		// These writes are all buffered by caller, so fine to do them