addReferenceDependency(name, version = '*') {
    if (!this._referenceDependencies) {
      this._referenceDependencies = {};
    }

    this._referenceDependencies[name] = version;
  }
Add a 'reference' entry to this object's `dependencies` list. A 'reference' dependency is
an entry in `dependencies` where the caller knows the dependency's source files are being
created elsewhere in the project tree, so no source files should be created locally in
`node_modules`, which is the standard FixturifyProject (and node-fixturify-project) behavior.
We do this by adding the necessary reference to `dependencies` during `toJSON`.

This is used when two addons wish to share a single definition on disk for a dependency (various parts of
ember-cli optimize processing based on paths on disk.)

Because there is no FixturifyProject being created, no callback is given as in other methods.

@param {String} name name of the dependency
@param {String} version version of the dependency, defaults to '*'. For our purposes, '*' means
"whatever version was specified elsewhere."