Support __dirname as a part of the build directory

[fsmaia]

• version:
• node version:
• npm (or yarn) version:

Do you want to request a feature or report a bug?:
Feature

What is the current behavior?:
Currently, there is a static buildDir (e.g. app/translations) which produces translations in a single file (one per locale).

What is the expected behavior?:
We could support <dirname> in the buildDir, so we could extract the messages to a JSON file inside the component folder.

For instance, if we have an app/components/Button/index.ts file, then its messages could be extracted to aop/components/Button/translations/en-US.json if we set the buildDir to <dirname>/translations.

Suggested solution:
Simply replacing <dirname> with __dirname.