Remove a few `Artifact` classes from `revng.support.artifact`. These
were needed to handle the idiosyncrasies of the legacy pipeline. Some of
the logic has been reworked into the ptml code.
The logic of the `revng ptml` command was too tightly coupled with the
parsing of the xml. Overhaul the structure of the ptml code and split it
in two locations:
* `revng.ptml`: this module contains functions that allow easy
manipulation of PTML, both for printing it and for obtaining the split
metadata/text version.
* `revng.internal.cli._commands.ptml`: this implements the actual `revng
ptml` command. This leverages the new logic in the `revng.ptml` module
while maintaining the same functionality.
Giacomo Vercesi