[tetrapak] exporting some useful functions for using it from a tetrapak plugins

Summary: Make the functions working not only on actual tetrapak path

Test Plan: read, compile

Reviewers: vtarasen, aschultz, hsychla, akuleshov, skattelmann, orixmann

Reviewed By: orixmann

CC: mkraemer, aschultz

Differential Revision: https://review.tpip.net/D1196