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/db/structs.go

@@ -5,7 +5,7 @@
 // You can obtain one at https://mozilla.org/MPL/2.0/.
 
 //go:generate go run ../../script/protofmt.go structs.proto
-//go:generate protoc -I ../../../../../ -I ../../vendor/ -I ../../vendor/github.com/gogo/protobuf/protobuf -I . --gogofast_out=. structs.proto
+//go:generate protoc -I ../../ -I . --gogofast_out=Mlib/protocol/bep.proto=github.com/syncthing/syncthing/lib/protocol:. structs.proto
 
 package db