Split code into modules

This is a massive commit that restructures the code into modules:

db/
    All functions related to modifying the Database

types/
    All type definitions and methods that can be exclusivly used on
    these types without dependencies

policy/
    All Policy related code, now without dependencies on the Database.

policy/matcher/
    Dedicated code to match machines in a list of FilterRules

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/app_test.go

@@ -1,75 +0,0 @@
-package hscontrol
-
-import (
-	"net/netip"
-	"os"
-	"testing"
-
-	"gopkg.in/check.v1"
-)
-
-func Test(t *testing.T) {
-	check.TestingT(t)
-}
-
-var _ = check.Suite(&Suite{})
-
-type Suite struct{}
-
-var (
-	tmpDir string
-	app    Headscale
-)
-
-func (s *Suite) SetUpTest(c *check.C) {
-	s.ResetDB(c)
-}
-
-func (s *Suite) TearDownTest(c *check.C) {
-	os.RemoveAll(tmpDir)
-}
-
-func (s *Suite) ResetDB(c *check.C) {
-	if len(tmpDir) != 0 {
-		os.RemoveAll(tmpDir)
-	}
-	var err error
-	tmpDir, err = os.MkdirTemp("", "autoygg-client-test")
-	if err != nil {
-		c.Fatal(err)
-	}
-	cfg := Config{
-		IPPrefixes: []netip.Prefix{
-			netip.MustParsePrefix("10.27.0.0/23"),
-		},
-		OIDC: OIDCConfig{
-			StripEmaildomain: false,
-		},
-	}
-
-	// TODO(kradalby): make this use NewHeadscale properly so it doesnt drift
-	app = Headscale{
-		cfg:      &cfg,
-		dbType:   "sqlite3",
-		dbString: tmpDir + "/headscale_test.db",
-
-		stateUpdateChan:       make(chan struct{}),
-		cancelStateUpdateChan: make(chan struct{}),
-	}
-
-	go app.watchStateChannel()
-
-	db, err := NewHeadscaleDatabase(
-		app.dbType,
-		app.dbString,
-		cfg.OIDC.StripEmaildomain,
-		false,
-		app.stateUpdateChan,
-		cfg.IPPrefixes,
-		"",
-	)
-	if err != nil {
-		c.Fatal(err)
-	}
-	app.db = db
-}