Store the outer filesystem's filename in a constant for later use

[johnpbloch]

This is related to (and required for) https://github.com/wp-cli/wp-cli/pull/5962

This will set an extra constant that stores the outer filesystem's filename. When that filename doesn't match with the phar's internal filename (wp-cli.phar), the phar can't find internal files that use __DIR__ for a reference point to the file.