demandimport: ignore `lzma` module for demandimport

This makes importing the module fail if the `_lzma` module is not present.

This makes e.g. tarfile correctly recognize if LZMA support is not present. It
changes the exception

    File "/usr/lib/python3.6/tarfile.py", line 1694, in xzopen
      fileobj = lzma.LZMAFile(fileobj or name, mode, preset=preset)
  AttributeError: module 'lzma' has no attribute 'LZMAFile'

to the more correct exception

    File "/usr/lib/python3.6/tarfile.py", line 1692, in xzopen
      raise CompressionError("lzma module is not available")
  tarfile.CompressionError: lzma module is not available

Also, it prevents that the error "abort: No module named '_lzma'!" is shown when
a development warning is to be shown. The reason why that happened is that for
showing the warning, we get information about the stack frames from the inspect
module, which accesses the `__file__` attribute of all modules in `sys.modules`
to build some cache, causing all modules (including `lzma`) to be imported.

hgdemandimport/__init__.py

@@ -1,81 +1,82
 # hgdemandimport - global demand-loading of modules for Mercurial
 #
 # Copyright 2017 Facebook Inc.
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 '''demandimport - automatic demand-loading of modules'''
 
 # This is in a separate package from mercurial because in Python 3,
 # demand loading is per-package. Keeping demandimport in the mercurial package
 # would disable demand loading for any modules in mercurial.
 
 from __future__ import absolute_import
 
 import os
 import sys
 
 if sys.version_info[0] >= 3:
     from . import demandimportpy3 as demandimport
 else:
     from . import demandimportpy2 as demandimport
 
 # Full module names which can't be lazy imported.
 # Extensions can add to this set.
 IGNORES = {
     '__future__',
     '_hashlib',
     # ImportError during pkg_resources/__init__.py:fixup_namespace_package
     '_imp',
     '_xmlplus',
     'fcntl',
     'nt',  # pathlib2 tests the existence of built-in 'nt' module
     'win32com.gen_py',
     'win32com.shell',  # 'appdirs' tries to import win32com.shell
     '_winreg',  # 2.7 mimetypes needs immediate ImportError
     'pythoncom',
     # imported by tarfile, not available under Windows
     'pwd',
     'grp',
     # imported by profile, itself imported by hotshot.stats,
     # not available under Windows
     'resource',
     # this trips up many extension authors
     'gtk',
     # setuptools' pkg_resources.py expects "from __main__ import x" to
     # raise ImportError if x not defined
     '__main__',
     '_ssl',  # conditional imports in the stdlib, issue1964
     '_sre',  # issue4920
     'rfc822',
     'mimetools',
     'sqlalchemy.events',  # has import-time side effects (issue5085)
     # setuptools 8 expects this module to explode early when not on windows
     'distutils.msvc9compiler',
     '__builtin__',
     'builtins',
     'urwid.command_map',  # for pudb
+    'lzma',
 }
 
 _pypy = '__pypy__' in sys.builtin_module_names
 
 if _pypy:
     # _ctypes.pointer is shadowed by "from ... import pointer" (PyPy 5)
     IGNORES.add('_ctypes.pointer')
 
 demandimport.init(IGNORES)
 
 # Re-export.
 isenabled = demandimport.isenabled
 disable = demandimport.disable
 deactivated = demandimport.deactivated
 
 
 def enable():
     # chg pre-imports modules so do not enable demandimport for it
     if (
         'CHGINTERNALMARK' not in os.environ
         and os.environ.get('HGDEMANDIMPORT') != 'disable'
     ):
         demandimport.enable()