demandimport: enforce ignore list while processing modules in fromlist

If a module is loaded as "from . import x" form, there has been no way to
disable demand loading for that module because name is ''. This patch makes
it possible to prevent demand loading by '<package-name>.x'.

We don't use _hgextimport(_origimport) here since attr is known to be a
sub-module name. Adding hgext_ to attr wouldn't make sense.

mercurial/demandimport.py

@@ -174,7 +174,12 b' def _demandimport(name, globals=None, lo'
             """
             symbol = getattr(mod, attr, nothing)
             if symbol is nothing:
-                symbol = _demandmod(attr, mod.__dict__, locals, level=1)
+                mn = '%s.%s' % (mod.__name__, attr)
+                if mn in ignore:
+                    importfunc = _origimport
+                else:
+                    importfunc = _demandmod
+                symbol = importfunc(attr, mod.__dict__, locals, level=1)
                 setattr(mod, attr, symbol)
 
             # Record the importing module references this symbol so we can