Detect externalModule dependencies inside subflow modules

Not sure this is 100% the right approach. If a subflow module has a dependency
it should be in the subflow's package.json and therefore installed next to the
subflow module in ~/.node-red/node_modules.

By treating it as a 'normal' external module, it will be dynamically installed
in ~/.node-red/externalModules. That then exposes the module to the user
who won't know why its there and may remove it.

It would be better to allow nodes inside a subflow module to require
from ~/.node-red/node_modules and not limit it to the externalModules
dir. The hard part is knowing when to do that.

packages/node_modules/@node-red/registry/lib/registry.js

@@ -456,6 +456,9 @@ function registerSubflow(nodeSet, subflow) {
         nodeSetInfo.config = result.config;
     }
     subflowModules[result.type] = result;
+    externalModules.registerSubflow(result.type,subflow);
+
+
     events.emit("type-registered",result.type);
     return result;
 }