Stop using Anod's langkit Python distribution and instead just ask users
to have their own Python 3.7+ distribution, and add a requirements.txt
file to install all Python dependencies in a single command.
TN: U302-007
Internal users are supposed to modify the `lkql_checker` program and its
rules only. There is thus no need to build liblkqllang (it’s installed
with Anod) and thus no need to install and setup Langkit.
TN: TB04-010