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/protocol/bep_extensions.go

@@ -1,7 +1,7 @@
 // Copyright (C) 2014 The Protocol Authors.
 
 //go:generate go run ../../script/protofmt.go bep.proto
-//go:generate protoc -I ../../vendor/ -I ../../vendor/github.com/gogo/protobuf/protobuf -I . --gogofast_out=. bep.proto
+//go:generate protoc -I ../../ -I . --gogofast_out=. bep.proto
 
 package protocol