build: add imports for go generate tools (#24682)

This adds a tools.go file to import all command packages used for
go:generate. Doing so makes it possible to execute go-based code
generators using 'go run', locking in the tool version using go.mod.

Co-authored-by: Felix Lange <fjl@twurst.com>

cmd/faucet/faucet.go

@@ -17,7 +17,7 @@
 // faucet is an Ether faucet backed by a light client.
 package main
 
-//go:generate go-bindata -nometadata -o website.go faucet.html
+//go:generate go run github.com/kevinburke/go-bindata/go-bindata -nometadata -o website.go faucet.html
 //go:generate gofmt -w -s website.go
 
 import (