* apt-pkg/contrib/fileutl.{cc,h}:

  - add GetModificationTime() helper
* apt-pkg/pkgcachegen.cc:
  - regenerate the cache if the sources.list changes to ensure
    that changes in the ordering there will be honored by apt
* apt-pkg/sourcelist.{cc,h}:
  - add pkgSourceList::GetLastModifiedTime() helper