[bugfix] Ensure TS blueprints are always used

[dfreeman]

This should be a quick 1.4.2 fix on the heels of 1.4.1 that just went out, which doesn't consistently emit .ts files from ember generate. It turns out init is not a safe place to do our project.addons munging, and ember-cli really isn't set up to make it simple to expose blueprints from another addon while also having your own.

Essentially:

• we can't sneak in the blueprints addon in init, because the project's own addons array is set after all its addons have been initialized
• we can't sneak in an additional blueprintsPath value because we need to also include our own, and every addon is allowed to return at most one
• the includedCommands hook is a convenient (if unfortunate) spot for us to use, as we know the addons will already have been initialized (so project.addons is populated) but it has to execute before blueprint paths are actually resolved (available commands are all loaded before any one is executed)

I've also added a test case to ensure this continues working with future releases of the CLI (though the codepaths involved have been pretty stable for some time). This isn't a beautiful solution, but I'm hoping to focus efforts on moving to 2.0 where we won't need to do this at all.

[jamescdavis]

💙