extraModules not being rebuilt from submodule

This may be related to issues #151 and #178 . Given a -w CLI flag or extraModules: option, native code modules in submodules are not rebuilt. I am using Electron 1.7.9 (also tested with 1.4.16) and electron-rebuild 1.6.0. I've attached a simple test project which can be built with:

npm install
npm run build (or npm run build-cli)

from the externalModules_Test directory (which links to the submodule_Test directory).

I believe that this used to work properly in a previous version of electron-rebuild, but I haven't had an opportunity to regression-test against previous versions. I am running OSX 10.13.1 and the test project is OSX-specific.

[extraModules_Tester.zip](https://github.com/electron/electron-rebuild/files/1535908/extraModules_Tester.zip)

I have forked and created a local fix in which the extraModules keys in prodDeps are set to true before the stuff in devKeys is added, but I am not familiar with the electron-rebuild code base and suspect that there's a better solution. In any case, my fix does solve the problem.

Start with the attached externalModules_Test project and its linked submodule_Test project. Run npm install followed by npm run build or npm run build-cli, then trace how the -w CLI flag and extraModules option handle native modules in submodules. Done means those native modules are rebuilt when either option is used.

Written by the indexing model from the issue text.