Split etcdctl into etcdctl (public API access) & etcdutl (direct surgery on files)

Motivation is as follows:

  - etcdctl we only depend on clientv3 APIs, no dependencies of bolt, backend, mvcc, file-layout
  - etcdctl can be officially supported across wide range of versions, while etcdutl is pretty specific to file format at particular version.
it's step towards desired modules layout, documented in: https://etcd.io/docs/next/dev-internal/modules/

scripts/test_lib.sh

@@ -164,7 +164,7 @@ function run_for_module {
 }
 
 function module_dirs() {
-  echo "api pkg raft client/pkg client/v2 client/v3 server etcdctl tests ."
+  echo "api pkg raft client/pkg client/v2 client/v3 server etcdutl etcdctl tests ."
 }
 
 # maybe_run [cmd...] runs given command depending on the DRY_RUN flag.
@@ -185,6 +185,7 @@ function modules() {
     "${ROOT_MODULE}/client/v2"
     "${ROOT_MODULE}/client/v3"
     "${ROOT_MODULE}/server/v3"
+    "${ROOT_MODULE}/etcdutl/v3"
     "${ROOT_MODULE}/etcdctl/v3"
     "${ROOT_MODULE}/tests/v3"
     "${ROOT_MODULE}/v3")