replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as
before. Notable exceptions are klog and gogo/protobuf.

vendor/golang.org/x/sys/unix/sockcmsg_unix.go

@@ -18,10 +18,13 @@ func cmsgAlignOf(salen int) int {
 	salign := SizeofPtr
 
 	switch runtime.GOOS {
-	case "darwin", "dragonfly", "solaris":
-		// NOTE: It seems like 64-bit Darwin, DragonFly BSD and
-		// Solaris kernels still require 32-bit aligned access to
-		// network subsystem.
+	case "aix":
+		// There is no alignment on AIX.
+		salign = 1
+	case "darwin", "dragonfly", "solaris", "illumos":
+		// NOTE: It seems like 64-bit Darwin, DragonFly BSD,
+		// illumos, and Solaris kernels still require 32-bit
+		// aligned access to network subsystem.
 		if SizeofPtr == 8 {
 			salign = 4
 		}