all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

lib/relay/protocol/packets.go

@@ -1,6 +1,6 @@
 // Copyright (C) 2015 Audrius Butkevicius and Contributors (see the CONTRIBUTORS file).
 
-//go:generate -command genxdr go run ../../../vendor/github.com/calmh/xdr/cmd/genxdr/main.go
+//go:generate -command genxdr go run ../../../repos/xdr/cmd/genxdr/main.go
 //go:generate genxdr -o packets_xdr.go packets.go
 
 package protocol