restore addon-import logic

[GavinJoyce]

Fixes #7726

In my attempt to simplify a quite complex piece of code while adding an additional condition, I inadvertently modified the logic such that mainBlueprint.supportsAddon() was called in more cases that before my change. It seems that calling blueprint.supportsAddon() has some side-effects (as it calls blueprint.files()) which resulted in some blueprint code executing without having a reference to this.project: #7726

Here is the original logic:

if (noAddonBlueprint.indexOf(name) < 0 && !addonBlueprint && (mainBlueprint && mainBlueprint.supportsAddon()) && options.args[1]) {
  addonBlueprint = this.lookupBlueprint('addon-import', true);
}

This is all quite brittle, but as this code will be removed once MU takes over the world, I think it's ok to leave as is.

[GavinJoyce]

CI seems to be an unrelated timeout:

[rwjblue]

Restarted that CI job