refactor(sidekick): move files to internal/sidekick/sidekick (#2818)

As a step toward consolidating the internal/sidekick/internal/ packages
under internal/sidekick, move the existing files into
internal/sidekick/sidekick.

Pure code in motion.

For #2816

Author: julieqiu

cmd/sidekick/main.go

@@ -18,7 +18,7 @@ import (
 	"fmt"
 	"os"
 
-	"github.com/googleapis/librarian/internal/sidekick"
+	"github.com/googleapis/librarian/internal/sidekick/sidekick"
 )
 
 func main() {