Runtime Exception Booted CiviCRM, but failed to find 'api/class.api.php'
It appears that civix is ignoring or overwriting additions to or modifications of the include_path. I added the full path name /Applications/MAMP/htdocs/xxx/civcrm/‌​wp-content/plug‌​ins/civicrm/‌​civicrm (where api resides) to include_path in the php.ini (checked I got the right one with php --ini). That didn't work. 

But I got it to work by soft linking the api directory into the new extension directory, i.e. changed directory to the extension directory and did the following command: $ ln -s /Applications/MAMP/htdocs/xxx/civcrm/‌​wp-content/plug‌​ins/civicrm/‌​civicrm/api .  

It's not an ideal solution but it enabled meet move forward.