demandimport: support "absolute_import" for external libraries (issue4029) Before this patch, demandimport of Mercurial may fail to load external libraries using "from __future__ import absolute_import": for example, importing "foo" in "bar.baz" module will load "bar.foo" if it exists, even though "absolute_import" is enabled in "bar.baz" module. So, extensions for Mercurial can't use such external libraries. This patch saves "level" of import request for on-demand module loading in the future: default value of level is -1, and level is 0 when "absolute_import" is enabled. "level" value is passed to built-in import function in "_demandmod._load()" and it should load target module correctly. This patch changes only one "_demandmod" construction case other than cases below: - construction in "_demandmod._load()" this code path should be used only in relative sub-module loading case - constructions other than patched one in"_demandimport()" these code paths shouldn't be used in "level != -1" case