mod+tools+Makefile:separate tools different module

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

tools/go.mod

@@ -0,0 +1,13 @@
+module github.com/lightningnetwork/lnd/tools
+
+go 1.16
+
+require (
+	github.com/btcsuite/btcd v0.22.0-beta.0.20211005184431-e3449998be39
+	github.com/dvyukov/go-fuzz v0.0.0-20210602112143-b1f3d6f4ef4e
+	github.com/elazarl/go-bindata-assetfs v1.0.1 // indirect
+	github.com/golangci/golangci-lint v1.18.0
+	github.com/ory/go-acc v0.2.6
+	github.com/rinchsan/gosimports v0.1.5
+	github.com/stephens2424/writerset v1.0.2 // indirect
+)