test-extdiff: fill in a missing Windows test

policy: eliminate ".pure." from module name only if marked as dual So we can switch cext/pure modules to new layout one by one.

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.

policy: mark all string literals as sysstr or bytes The policy module won't be imported early in future, which means string literals will be processed by our Python 3 loader.

debuginstall: check C extensions only if they are loadable per policy This check is useless in pure installation and I want to make it directly import C extension modules.

osutil: proxy through util (and platform) modules (API) See the previous commit for why. Marked as API change since osutil.listdir() seems widely used in third-party extensions. The win32mbcs extension is updated to wrap both util. and windows. aliases.

mpatch: proxy through mdiff module See the previous commit for why.

bdiff: proxy through mdiff module See the previous commit for why. mdiff seems a good place to host bdiff functions. bdiff.bdiff was already aliased as textdiff, so we use it.