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

.gitignore

@@ -23,3 +23,4 @@ parts/
 stage/
 *.snap
 *.bz2
+/repos