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/syscall.go

@@ -50,5 +50,4 @@ func BytePtrFromString(s string) (*byte, error) {
 }
 
 // Single-word zero for use when we need a valid pointer to 0 bytes.
-// See mkunix.pl.
 var _zero uintptr