check: check modules in hgdemandimport A few places only check modules in mercurial and hgext. Add hgdemandimport to the list in those places.

demandimport: move to separate package In Python 3, demand loading is per-package. Keeping demandimport in the mercurial package would disable demand loading for any modules in mercurial.

import-checker: add a way to directly import certain symbols We'll use this for the 'demandimport' symbol in an upcoming patch.

check-code: allow skipping hasattr check in py3-only code hasattr is safe in Python 3, and in an upcoming patch we can't use util.safehasattr.

profiling: allow loading profiling extension before everything else 6d642ecf1a89 makes profiler start early without loading extensions. That makes it impossible for an extension to add customized profilers. This patch adds a special case: if a profiler is not found but an extension with the same name could be loaded, load that extension first, and expect it to have a "profile" contextmanager method. This allows customized profilers and extension setup time is still profiled.

extensions: allow loading a whitelisted subset of extensions This feature will be used by the next patch.