Restructure non-Avogadro specific code into a package that can be published on PyPI

[matterhorn103]

Each of the modules in the plugin can be conceptually divided into two parts: functions that provide essentially an API to xtb/crest, and a __main__ function that provides an entry point for use by Avogadro.

The API-like functions are actually useful outside of Avogadro as well, for setting up, running, and processing the results of calculations with xtb or crest. If a neat division can be achieved between the two aspects of avo_xtb, the API could be uploaded to PyPI for use elsewhere.

The aim would not be to provide a complete or performant API to xtb, especially given the existence of the tblite project, but it only seems sensible to make the functions available for general use.