[PATCH 0 of 1 V2] demandimport: allow extensions to import own modules by absolute name

[FUJIWARA Katsunori]

This patch allows modules of extensions to import another one in own
extension by absolute name.

Difference from V1 is that test extension is located into other than
$PWD.

Recent (on "default" branch) hgsubversion extension is a one of
examples using absolute import. It can't work correctly when it is
enabled in usual way "[extensions] hgsubversion=path/to/hgsubversion",
because absolute import fails.

  https://bitbucket.org/durin42/hgsubversion/

  reported issues:
    https://bitbucket.org/durin42/hgsubversion/issue/399/relative-import-should-be-used
    https://bitbucket.org/durin42/hgsubversion/issue/402/getting-abort-no-module-named-hgsubversion