- probably breaks a lot of stuff; c++ compilers, distcc, possibly others
- split into late_prepare_host_dependencies / early_prepare_host_dependencies calling adaptative_prepare_host_dependencies() full of if's
also:
- calc deps out of configuration
- reorg checks into obtain_and_check_host_release_and_arch()
- completely remove support for building under `buster` -- that's way too old, sorry.
- de-hardcode `python3` invocations, instead use `python3_binary_path` set by `prepare_python3_binary_for_python_tools()`
- juggle `$HOSTRELEASE`: read from actual host, or determined from Docker image name (during Dockerfile build)
- TL;DR: include and use `python3.9` for focal-like host OS's