bpo-36146: Refactor setup.py: Add PyBuildExt.srcdir (GH-12124)

* Add PyBuildExt.srcdir atribute in setup.py: the source directory is
  now always absolute.
* Add PyBuildExt.inc_dirs and PyBuildExt.lib_dirs attributes:
  replace 'inc_dirs' and 'lib_dirs' local variables of
  detect_modules().
* Replace "from distutils.errors import *"
  with "from distutils.errors import CCompilerError, DistutilsError"
  to be able to use static analyzers like pyflakes
* Reorder imports.