[bugfix] Allow VersionChecker to work with Projects

[pzuraq]

There are two main use cases for VersionChecker today:

1. Checking the version of an NPM dependency of an Addon
2. Checking the version of an NPM or Bower dependency of the root Project

These use cases are currently intermingled in the VersionChecker API, which causes some confusing behavior:

1. When passing an Addon to a VersionChecker:

   • Checking an NPM dependency will check the addon's package.json directly, but checking a Bower dependency will check the root Project's bower.json. There is no way to check the root Project's NPM dependencies.

   • Attempting to use forEmber will attempt to check the Addon's version of ember-source (which shouldn't exist), and if it is not found will attempt to check the Bower dependencies of the root Project. This forking of behavior is confusing because it correctly detects the version of Ember for users who are still using Bower, but will not for users who switch to ember-source.

     The "fix" for this is to use _findHost to find the root EmberApp and to pass that to VersionChecker instead, but this breaks as described below and is still confusing (why can I even call forEmber on a VersionChecker for a parent addon when it has this behavior?)

2. When passing a Project to a VersionChecker:

   • It will error because the Bower checker assumes that it will receive an item with a project property which is a Project.

3. When passing an EmberApp to a VersionChecker:

   • It will attempt to check NPM dependencies using an undefined root, but will correctly check Bower dependencies. This generally works, but breaks when using yarn link because NPM's module resolution defaults to the addon which is using VersionChecker rather than the Project root.

This PR proposes that we should clarify the API by enforcing that users explicitly pass the thing they are trying to check the version of, either an Addon or a root Project, to VersionChecker:

included(parent) {
  this._super.included.apply(this, arguments);

  // Currently the only meaningful way to check if the parent is an Addon or an EmberApp
  if (parent.root !== undefined) {
    let parentAddonChecker = new VersionChecker(parent);
  
    parentAddonChecker.forEmber(); // Should throw an error, doesn't make sense
    parentAddonChecker.for('foo', 'bower'); // Should throw an error, doesn't make sense
    parentAddonChecker.for('foo', 'npm'); // allowed
  }

  let rootVersionChecker = new VersionChecker(this.project);

  parentAddonChecker.forEmber(); // allowed
  parentAddonChecker.for('foo', 'bower'); // allowed
  parentAddonChecker.for('foo', 'npm'); // allowed
}

We would do this by first allowing VersionChecker to accept a Project directly (as this PR does), adding deprecations, and allowing users to switch over to the new usage style, then creating a new major version.

[rwjblue]

The description infers this affects npm and bower alike, but I only see changes to the bower dependency checker. Am I misunderstanding something or is there a missing change?

[pzuraq]

So, the real blocking issue here is that the Bower dependency checker always assumes there will be a project property on whatever we pass to VersionChecker. So if I pass a Project directly, it'll throw an error whenever we're using forEmber and it doesn't find ember in the NPM deps (or whenever we attempt to find the Bower deps).

However, if we pass an EmberApp, it doesn't have a root property, so the NPM checker doesn't check the correct directory. It's a catch 22 🙃

In order to correctly check an Ember app's Ember version today, you would have to do something like this:

included() {
  let project = this.project;

  let checker = new VersionChecker({ root: project.root, project });
}