Fix package list inconsistency caused by namespace package on Python 3.5

Unload all pkg_resources modules and not just the main module.

Fix EBNF for package specifications.

fix an issue for bdist_wininst with gui_scripts:

update certifi to 2015.11.20

The script header was always generated with non-gui executable.
This was caused by by adjust_header assuming the executable is always an
absolute path.

Fixed by using find_executables() from distutils.

--HG--
branch : bdist_wininst_gui_scripts

--HG--
branch : sync_certifi

namespace package will be skipped during installation. Since Python 3.5, .pyo files are removed and new .opt-1.pyc (and .opt-2.pyc) files are introduced [1]. However setuptools does not understand that new naming therefore the corresponding foo.opt-1.pyc is still added into package list (via --record). The inconsistency leads to a packaging error.

[1] https://www.python.org/dev/peps/pep-0488/