Ensure external helpers are not used when modules are not transpiled.

[rwjblue]

When using something akin to the following somewhat common pattern:

let babel = this.addons.find(addon => addon.name === 'ember-cli-babel');
let tree = babel.transpileTree(input, {
  'ember-cli-babel': {
    compileModules: false,
  }
});

The previous logic would still attempt to use external helpers even when modules themselves are not being transpiled. This meant that the final output would include untranspiled import statements.

This affects common addons such as:

• ember-fetch
• ember-service-worker
• ember-data
• @ember/test-helpers
• ember-angle-bracket-invocation-polyfill
• ember-modifier-manager-polyfill

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The fix here is to detect that modules will not be compiled, and avoid usage of the external helpers.

This also moves the validation that parent addon's do not specify includeExternalHelpers in their own config to the shared getAddonConfig method (and makes the logic in that method a bit easier to grok).

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Fixes ember-polyfills/ember-modifier-manager-polyfill#4