[PATCH 0 of 3 V3] demandimport: improve demandimport for extensions/external libraries

[FUJIWARA Katsunori]

This patch series improves demandimport for extensions/external libraries.

Difference from V2:

  - add patch #1, which adds "absimport" feature to hghave
  - add patch #2, which fixes issue4029 ("absolute_import" support)
  - adjust patch #3 to apply it on patch #2

This series also resolves problems of some 3rd party extensions, for
example:

  - reported for mercurial_keyring (fixed by patch #2)

    - https://bitbucket.org/Mekk/mercurial_keyring/issue/31/failed-to-import-extension

      I confirmed that patch #2 fixes this issue with sources of
      python-keyring-lib 3.0.1: recent python-keyring-lib itself
      includes changes to fix this issue, in fact.

        https://bitbucket.org/kang/python-keyring-lib/issue/112/after-upgrading-to-the-latest-version-it

  - reported for hgsubversion (fixed by patch #3)

    - 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