mercurial: don't load C extensions from PyPy PyPy isn't compatible with Python C extensions. With this patch, the module load policy is automatically to "Python only" when run under PyPy. `hg` and other Python scripts importing mercurial.* modules will run from the source checkout or any installation when executed with PyPy. This should enable people to more easily experiment with PyPy and its potentially significant performance benefits over CPython!

mercurial: be more strict about loading dual implemented modules With this change in place, we should have slightly stronger guarantees about how modules with both Python and C implementations are loaded. Before, our module loader's default policy looked under both mercurial/* and mercurial/pure/* and imported whatever it found, C or pure. The fact it looked in both locations by default was a temporary regression from the beginning of this series. This patch does 2 things: 1) Changes the default module load policy to only load C modules 2) Verifies that files loaded from mercurial/* are actually C modules This 2nd behavior change makes our new module loading mechanism stricter than from before this series. Before, it was possible to load a .py-based module from mercurial/*. This could happen if an old installation orphaned a file and then somehow didn't install the C version for the new install. We now detect this odd configuration and fall back to loading the pure Python module, assuming it is allowed. In the case of a busted installation, we fail fast. While we could fall back, we explicitly decide not to do this because we don't want people accidentally not running the C modules and having slow performance as a result.