Replace docserv-stitch with Python code

[tomschr]

Situation

The scriptdocserv-stitch is a separate script and is called inside Docserv. The script validates productconfig files (Docserv configs) from a directory one by one, then stitch them together so Docserv can build it.

This design has some issues:

• It's inefficient as the internal state needs to be passed to the script (that's why it takes many options)
• Bash is slower than pure Python code.

Proposed solution

• Integrate the functionality into Docserv code as Python.
• If really needed, provide a Python script that calls the internal code.