python3: allow hgloader to work with lazy loaders Don't clobber the loader returned from find_spec. This brings `hg version` down from 0.27 seconds to 0.17.

init: turn on demandimport for Python 3.6 and above This uses the new demandimport implementation for Python 3 introduced in previous patches. This doesn't yet enhance performance because it isn't integrated with the custom source file loader we use on Python 3. We'll integrate the two in upcoming patches.

demandimport: add python 3 implementation This implementation uses the new importlib finder/loader functionality available in Python 3.5 and up. # no-check-commit

demandimport: move ignore list to __init__.py We're going to use the same ignore list for Python 3.

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.