Provide an alternative to `setup.py` commands for Linux packaging

[t20100]

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As it is now, Linux/Debian packaging relies on 2 setup.py commands:

• debian_src: To generate a source tarball with some specific stripping, mostly (and probably only) in the third_party sub-module.
• build_man: To build the man page.

Since setuptools is deprecating direct call of commands, what should we use instead? For the build_man, I tried to make a standalone script for it... but it only works if the package is installed, which is not the case during Debian packaging process.

Should we add some environment variables to control the build process (i.e., what submodule to strip, whether to build man page or not)?

attn @kif @picca