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/subflow.json

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