Use realpath(__file__) to locate whole portage.

Use realpath(__file__) so that python module symlinks in site-packages
are followed back to the real location of the whole portage
installation.
portage.const: realpath(__file__.rstrip("co"))

With python2.x, __file__ may refer to the compiled bytcode file, but we
want to follow the symlink of the uncompile py file.