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/text/unicode/bidi/gen.go

@@ -26,7 +26,7 @@ func main() {
 }
 
 // bidiClass names and codes taken from class "bc" in
-// http://www.unicode.org/Public/8.0.0/ucd/PropertyValueAliases.txt
+// https://www.unicode.org/Public/8.0.0/ucd/PropertyValueAliases.txt
 var bidiClass = map[string]Class{
 	"AL":  AL,  // ArabicLetter
 	"AN":  AN,  // ArabicNumber