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.

test/resources/subflow/package/package.json

@@ -1,6 +1,6 @@
 {
   "name": "test-subflow-mod",
-  "version": "1.0.1",
+  "version": "1.0.2",
   "description": "",
   "keywords": [],
   "license": "ISC",
@@ -13,6 +13,7 @@
       ]
   },
   "dependencies": {
-      "node-red-node-random": "*"
+      "node-red-node-random": "*",
+      "cowsay2": "*"
   }
 }

test/resources/subflow/package/subflow.json

@@ -189,6 +189,7 @@
             "noerr": 0,
             "initialize": "",
             "finalize": "",
+            "libs": [ {"var":"cowsay2","module":"cowsay2"}],
             "x": 240,
             "y": 100,
             "wires": [