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.

setup: refactor handling of modules with C/Python implementations Previously, .py files under mercurial/pure/ were copied to mercurial/* during installation if we were performing a pure Python installation. Now that the new import hooks and module load policy are in place, this hackery from the past is no longer necessary. With this patch, we stop copying modules from mercurial/pure/* to mercurial/*. Instead, we preserve the files at their original hierarchy, mirroring the source repository structure. In addition, we always install the pure modules. Before, we would only include the pure modules in the distribution/installation if the install-time settings requested a pure Python installation. The upside of this change is that CPython and PyPy can run from the same Mercurial installation, making packaging and distribution of Mercurial simpler. The inclusion of pure Python modules in the installation sounds risky, as it could lead to inadvertent loading of non-C modules. This shouldn't be a problem. The default module load policy is "C only" (or at least will be shortly) and the only way to load pure modules from an installation is if a) pure installation was requested b) the HGMODULELOADPOLICY overrides the requirement for C modules. The default module load policy as defined in source is a special string whose default value from the checkout is equivalent to the "C only" policy (again, not exactly the state right now). For pure installations, this default policy is not appropriate and will not work. This patch adds support for rewriting __init__.py during installation to reflect the module load policy that should be in place accoding to the installation settings. For default CPython installs, the value in the source file will change but there will be no functional change. For pure installations, the default policy will be set to "py," allowing them to work without having to set environment variables.

check-seclevel: set module load policy to Python only If we don't change this, the upcoming change to make the module loading policy only load C modules will cause this script to fail if run with CPython against an unbuilt source checkout.