build: enable goimports and varcheck linters (#16446)

Makefile

accounts/usbwallet/ledger.go

build/ci.go

build/goimports.sh

cmd/ethkey/main.go

consensus/consensus.go

core/asm/compiler.go

core/tx_pool.go

core/types/transaction.go

core/vm/instructions.go

crypto/bn256/cloudflare/example_test.go

les/bloombits.go

les/serverpool.go

les/sync.go

p2p/discover/udp.go

p2p/discv5/net.go

p2p/discv5/nodeevent_string.go

p2p/discv5/table.go

p2p/discv5/udp.go

p2p/discv5/udp_test.go