[Meta issue] List of cpt.py improvements

[reikdas]

• 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: Fix deb creation vgvassilev/cling#214) [Clarify why there is no packages #307]
• 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 [cpt.py disfunctioning #372]
• Use homebrew as default package manager on macOS, instead of macports [[cpt.py] --check-requirements doesn't detect Homebrew as a package manager and misdetect libz (zlib) dependecy #404]
• Check if installed dependencies are always detected by --check-requirements (even if installed by different package manager); also verify list of required dependencies [[cpt.py] --check-requirements doesn't detect Homebrew as a package manager and misdetect libz (zlib) dependecy #404]
• New flag to specify number of cores to build Cling with [Add option in cpt.py to control the number of cores used during make #391]

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