Add support to helper.py for external projects for all subcommands except `download_corpora`.
External users just need to specify `build_integration_path`, `project_src_path` and `external`.
Also do a big refactor to pass a project object as part of args instead of a string containing the name of the project or its
path.
Related: #6125
Fix behavior of build_image --pull (Fixes#5640)
Also refactor helper.py:
1. Change behavior of functions so that most return True on success and False on failure.
2. Only main will return 1 on failure and 0 on success now. Previous behavior was very error prone.
3. Rename _get_output_dir to _get_out_dir.
4. Make function docstrings use descriptive tense.
5. Make helper.py print help when no argument is specified.
jonathanmetzman