[Meta issue] List of `cpt.py` improvements

[reikdas]

• [x] Use a if __name__ == "__main__" block as program execution starting point in cpt.py
• [ ] No mutating global variables in cpt.py
• [ ] Minimize use of subprocess in cpt.py
• [ ] Making cpt.py flake8 compliant (with appropriate error/violation codes)
• [ ] Fix .deb packaging (existing effort: https://github.com/vgvassilev/cling/pull/214) [#307]
• [x] Fix --verbose flag not printing verbose output in CMake build log (using cmake -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON instead of make VERBOSE=1 might fix this)
• [ ] Do not allow users to use currently "known" broken/untested features in cpt.py; comment out relevant parts
• [ ] Revamp argument parser
  • cpt.py is both a Cling installer and a packager. It should be possible to build Cling without packaging it.
  • Enable dependent arguments. Ideally, we should be able to pass some arguments only if some other argument is also passed (for eg. it doesn't make sense to pass --with-llvm-url if LLVM is being installed from system package manager).
  • Fix nomenclature (we have arguments named --with-llvm-tar and --with-binary-llvm; there should be some uniformity)
• [ ] Fix builds using LLVM pre-built binary tarballs (on Linux and macOS) and add it to CI
• [ ] Fix builds using LLVM installed from system package manager (on Linux and macOS) and add it to CI
• [ ] Enable builds using LLVM pre-built binaries on Windows and add it to CI
• [ ] Update cpt.py README, both in root directory as well as in tools/packaging [#372]
• [ ] Use homebrew as default package manager on macOS, instead of macports [#404]
• [ ] Check if installed dependencies are always detected by --check-requirements (even if installed by different package manager); also verify list of required dependencies [#404]
• [ ] New flag to specify number of cores to build Cling with [#391]

(Please check off closed issues and add new improvements by editing this top post)