test-extdiff: deduplicate tests

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.