derp/derpserver: split off derp.Server out of derp into its own package

This exports a number of things from the derp (generic + client) package
to be used by the new derpserver package, as now used by cmd/derper.

And then enough other misc changes to lock in that cmd/tailscaled can
be configured to not bring in tailscale.com/client/local. (The webclient
in particular, even when disabled, was bringing it in, so that's now fixed)

Fixes #17257

Change-Id: I88b6c7958643fb54f386dd900bddf73d2d4d96d5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derphttp/export_test.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package derphttp
+
+func SetTestHookWatchLookConnectResult(f func(connectError error, wasSelfConnect bool) (keepRunning bool)) {
+	testHookWatchLookConnectResult = f
+}
+
+// breakConnection breaks the connection, which should trigger a reconnect.
+func (c *Client) BreakConnection(brokenClient *Client) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.client != brokenClient.client {
+		return
+	}
+	if c.netConn != nil {
+		c.netConn.Close()
+		c.netConn = nil
+	}
+	c.client = nil
+}
+
+var RetryInterval = &retryInterval