reloadProjects() {
    let projects = this.projects.slice();
    this._clear();
    projects.forEach((project) => this.loadProject(project));
  }
To support the project.reloadPkg method, we need the ability to flush
the cache and reload from the updated package.json.
There are some issues with doing this:
  - Because of the possible relationship between projects and their addons
    due to symlinks, it's not trivial to flush only the data related to a
    given project.
  - If an 'ember-build-cli.js' dynamically adds new projects to the cache,
    we will not necessarily get called again to redo the loading of those
    projects.
The solution, implemented here:
  - Keep track of the Project objects whose packages are loaded into the cache.
  - If a project is reloaded, flush the cache, then do loadPackage again
    for all the known Projects.

@public
@method reloadProjects
@return null