merge with apt/experimental

merge with apt/experimental

* apt-pkg/packagemanager.cc:
  - fix bug in predepends handling - ensure that packages that needs
    unpackaging are unpacked before they are configured (LP: #927993)
[ Julian Andres Klode ]
* apt-pkg/deb/deblistparser.cc:
  - Set the Essential flag on APT instead of only Important
* apt-pkg/packagemanager.cc:
  - Do not use immediate configuration for packages with the Important flag
* Treat the Important flag like the Essential flag with those differences:
  - No Immediate configuration (see above)
  - Not automatically installed during dist-upgrade
  - No higher score for installation ordering