s to APTs idea of the
list of available packages.

The cache can be used like a mapping from package names to Package
objects (although only getting items is supported).

Keyword arguments:
progress -- a OpProgress object,
rootdir  -- an alternative root directory. if that is given the system
sources.list and system lists/files are not read, only file relative
to the given rootdir,
memonly  -- build the cache in memory only.


.. versionchanged:: 1.0

    The cache now supports package names with special architecture
    qualifiers such as :all and :native. It does not export them
    in :meth:`keys()`, though, to keep :meth:`keys()` a unique set.
Nc