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

vendor/golang.org/x/sys/windows/syscall.go

@@ -11,11 +11,14 @@
 // system, set $GOOS and $GOARCH to the desired system. For example, if
 // you want to view documentation for freebsd/arm on linux/amd64, set $GOOS
 // to freebsd and $GOARCH to arm.
+//
 // The primary use of this package is inside other packages that provide a more
 // portable interface to the system, such as "os", "time" and "net".  Use
 // those packages rather than this one if you can.
+//
 // For details of the functions and data types in this package consult
 // the manuals for the appropriate operating system.
+//
 // These calls return err == nil to indicate success; otherwise
 // err represents an operating system error describing the failure and
 // holds a value of type syscall.Errno.