NPM_PACKAGE_DIRS to mimick a local package directory

[eric-burel]

Is your feature request related to a problem? Please describe.
Mimick Meteor beloved METEOR_PACKAGE_DIRS feature in NPM.

• When this variable is set, Meteor will consider each directory listed as a registry package. It will fetch packages from there in priority, and fetch on Atmosphere otherwise
• When it is not set, it goes back to getting the packages from Atmosphere
• This behaviour happens at runtime => it means we would need a vulcan CLI command (like vercel, blitz already do for next)

Describe the solution you'd like

Run development mode like this:
NPM_PACKAGE_DIRS=<your-vulcan-npm-copy> vulcan dev
And so on for each next command that may rely on NPM packages.

Describe alternatives you've considered

• yalc for the setup. Commands are already documented in the docs.
• See this ticket: Does Yalc resolve dependencies as if I npm installed the local module? wclr/yalc#50. Yalc seems to also need an install command afterward? To be confirmed.
• https://github.com/google/zx for writing scripts easily in JS if we need to run commands? Or we might need a more cross-plateform alternative like https://www.npmjs.com/package/shelljs
• • Verdaccio might be something to dig, as it spawns an actual NPM registry locally

Additional context

• More explanation about why we need this feature, with the example of React: Hooks don't work with yarn link facebook/react#14257
• Meteor doc: https://docs.meteor.com/environment-variables.html