Rename the module to the Coffey-Labs organisation

The repositories moved off LINUXexpert-org. Here that is not a
documentation change: the old organisation was the module path, so it is
declared in go.mod and repeated in every internal import.

Leaving it would have been worse than a stale link. GitHub redirects the
repository, but a go.mod whose module line disagrees with the path it was
fetched from is an error rather than a redirect, so `go get` on the new
address would have failed against the old declaration.

go.mod, 34 files of imports, and the repository links in README and
ARCHITECTURE. go mod tidy leaves go.sum untouched -- no dependency moved,
only our own path.

internal/backup/backup.go

@@ -11,7 +11,7 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/LINUXexpert-org/stalwart-migrator/internal/checkpoint"
+	"github.com/Coffey-Labs/stalwart-migrator/internal/checkpoint"
 )
 
 // Options configures a full backup pass. Which fields matter depends on

internal/backup/backup_test.go

@@ -12,7 +12,7 @@ import (
 	"path/filepath"
 	"testing"
 
-	"github.com/LINUXexpert-org/stalwart-migrator/internal/checkpoint"
+	"github.com/Coffey-Labs/stalwart-migrator/internal/checkpoint"
 )
 
 func TestBackupRunEndToEndAndResume(t *testing.T) {