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