mod+tools+Makefile:separate tools different module

Separated the tools to different go module to avoid having tools
dependencies on the main module.

build/dependencies.go

@@ -0,0 +1,15 @@
+//go:build buildtagdoesnotexist
+// +build buildtagdoesnotexist
+
+package build
+
+// This file is a workaround to make sure go mod keeps around the btcd and fuzz
+// dependencies in the go.sum file that we only use during certain tasks (such
+// as integration tests or fuzzing) or only for certain operating systems. For
+// example, the specific btcd import makes sure the indirect dependency
+// github.com/btcsuite/winsvc is kept in the go.sum file. Because of the build
+// tag, this dependency never ends up in the final lnd binary.
+import (
+	_ "github.com/btcsuite/btcd"
+	_ "github.com/dvyukov/go-fuzz/go-fuzz-dep"
+)