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/evm/internal/t8ntool/block.go

@@ -36,7 +36,7 @@ import (
 	"gopkg.in/urfave/cli.v1"
 )
 
-//go:generate gencodec -type header -field-override headerMarshaling -out gen_header.go
+//go:generate go run github.com/fjl/gencodec -type header -field-override headerMarshaling -out gen_header.go
 type header struct {
 	ParentHash  common.Hash       `json:"parentHash"`
 	OmmerHash   *common.Hash      `json:"sha3Uncles"`