policy: add "cext" package which will host CPython extension modules

I'm going to restructure cext/pure modules and get rid of our hgimporter
hack. C extension modules will be moved to cext/ directory so old and new
compiled modules can coexist in development tree. This is necessary to
run 'hg bisect' without recompiling.

New extension modules will be loaded by an importer function:

  base85 = policy.importmod('base85')  # select pure.base85 or cext.base85

This will also allow us to split cffi from pure modules, which is currently
difficult because pure modules can't be imported by name.

setup.py

@@ -577,7 +577,10 b" cmdclass = {'build': hgbuild,"
             'build_hgexe': buildhgexe,
             }
 
-packages = ['mercurial', 'mercurial.hgweb', 'mercurial.httpclient',
+packages = ['mercurial',
+            'mercurial.cext',
+            'mercurial.hgweb',
+            'mercurial.httpclient',
             'mercurial.pure',
             'hgext', 'hgext.convert', 'hgext.fsmonitor',
             'hgext.fsmonitor.pywatchman', 'hgext.highlight',
@@ -586,6 +589,7 b" packages = ['mercurial', 'mercurial.hgwe"
 common_depends = ['mercurial/bitmanipulation.h',
                   'mercurial/compat.h',
                   'mercurial/util.h']
+common_include_dirs = ['mercurial']
 
 osutil_cflags = []
 osutil_ldflags = []
@@ -615,21 +619,27 b" if sys.platform == 'darwin':"
 
 extmodules = [
     Extension('mercurial.base85', ['mercurial/base85.c'],
+              include_dirs=common_include_dirs,
               depends=common_depends),
     Extension('mercurial.bdiff', ['mercurial/bdiff.c',
                                   'mercurial/bdiff_module.c'],
+              include_dirs=common_include_dirs,
               depends=common_depends + ['mercurial/bdiff.h']),
     Extension('mercurial.diffhelpers', ['mercurial/diffhelpers.c'],
+              include_dirs=common_include_dirs,
               depends=common_depends),
     Extension('mercurial.mpatch', ['mercurial/mpatch.c',
                                    'mercurial/mpatch_module.c'],
+              include_dirs=common_include_dirs,
               depends=common_depends),
     Extension('mercurial.parsers', ['mercurial/dirs.c',
                                     'mercurial/manifest.c',
                                     'mercurial/parsers.c',
                                     'mercurial/pathencode.c'],
+              include_dirs=common_include_dirs,
               depends=common_depends),
     Extension('mercurial.osutil', ['mercurial/osutil.c'],
+              include_dirs=common_include_dirs,
               extra_compile_args=osutil_cflags,
               extra_link_args=osutil_ldflags,
               depends=common_depends),