If a Meteor package had a file called index.js, the runtime module system
would resolve "meteor/<name>" to "/node_modules/meteor/<name>/index.js",
instead of falling back to Package[<name>] as expected.
Installing a stub for Package[<name>] at /node_modules/meteor/<name>.js
means the runtime module system no longer needs the fallback, and will no
longer be confused by index.js files.
Fixes#6590.
The implementation of these stubs is controlled by an optional npm package
called meteor-node-stubs, so we can keep iterating on them after releasing
Meteor 1.3, if we need to.
Fixes#6056.
