Include addon name directory in 'treeForAddon' (mitigate issues with Embroider when using external helpers)

[charlespierce]

Info

• The default treeForAddon method provided by ember-cli ensures that all compiled assets are put into a directory named after the addon itself (see https://github.com/ember-cli/ember-cli/blob/9911f5493c00986ac2d0be104a80152426240a30/lib/models/addon.js#L1196)
• With its custom treeForAddon method, however, ember-cli-babel currently doesn't do that for the babel helpers.
• This can cause issues when the build system (e.g. Embroider) expects that behavior from an addon.

Changes

• Added a Funnel that includes the addon module name directory when building the Babel helpers (using the same moduleName method that the base addon class uses.