Reduces the number of calls to get_package_type.

get_package_type in the package controller is called at the start of
several functions to retrieve the type field of the package - this is
done with a direct database call.  Most of these methods will go on to
call package_show before needing the value of package_type, and so this
PR removes lots of the calls and replaces them with pkg_dict accesses
closer to the where it is needed.

_get_package_type comment has also changed to more closely reflect what
it actually does (rather than what we wish it did).

This should close #2080
Merge pull request #2084 from ckan/2080-reduce-use-of-get-package-type

Reduces the number of calls to get_package_type.