Simplify install_helpers.py

[rolyp]

Strip all the extraneous cruft out of install_helpers.py. Started at 936 LOC; currently 227.

• [x] purge (obsolete) __author__, __date__ and __version__ metadata
• [x] remove silly self argument documentation
• [x] delete __run_checks()
  • [x] remove (redundant) Python version-checking
  • [x] remove (unused) sys.argv[1] == ‘install’ behaviour
• [x] package_abs_root shenigans
  • [x] setter not required – remove use in setup.py and initialise inside InstallationConfiguration.__init__
  • [x] getter not required (unused)
• [x] get_platform shenanigans
  • [x] inline get_platform
  • [x] delete OTHER platform
  • [x] purge Enum nonsense
• [x] delete message_printer debacle – only used in ComponentChecker
  • [x] make MESSAGE_PREFIX local
• [x] delete (unused) ComponentChecker shenanigans
  • [x] delete (unused) check_libraries insanity
• [x] include_dirs shenanigans
  • [x] don’t even need InstallExtensionCommand
  • [x] delete CPATH treatment
• [x] library_dirs shenanigans
  • [x] delete library_dirs setter
  • [x] delete (unused) __library_dirs field