Update _shouldHandleTypeScript to read from pkg.dependencies instead of relying on addons

[brendenpalmer]

This fixes a bug between ember-cli-typescript, ember-engines dedupe, and ember-cli-babel. ember-cli-babel uses the existence of ember-cli-typescript as a dependency to determine whether to process *.ts files; however, ember-engines dedupe is stateful so it's possible for ember-cli-typescript to not be a dependency (part of addons) when ember-cli-babel is running.

For more info on ember-engines dedupe logic: https://github.com/ember-engines/ember-engines/blob/master/packages/ember-engines/lib/utils/deeply-non-duplicated-addon.js#L35