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.
The exec and events components are common components that
are used by both runtime and registry. It makes sense to
move them into the util package.
This also adds some docs to the registry module
