Empty packages as valid v2 addons

[BlueCutOfficial]

The issue was initially raised by #1931.

Problem

In tests relying on ember-data 4.4, the rewritten-packages contain multiple copies of @ember-data/store. Only one copy has content, the others are empty. The only copy having content is the only "active" package.

During the build process, we expect requests for the empty packages to resolve to the corresponding "active" package; but instead, they resolved to the empty copy, which triggered a build error.

Cause

This was because when the core module-resolver handles an empty package, it returns before the rehoming to the active package can happen. The condition that triggered the return was: if (!pck.isV2Addon()) return /* ... */: in other words, empty packages generated by the EmptyPackageTree plugin are not valid v2 addons, causing the module-resolver to give an unexpected answer.

Solution

This PR changes the EmptyPackageTree plugin to add more information in the package.json of the empty package: it now has the keys that identify a package as a valid v2 package.

Once the module-resolver recognizes an empty package as a valid v2 package, it continues the execution until it rehomes the request and resolve to the active package.

[mansona]