Move package autocomplete from package controller and move to API

[rgrp]

Currently autocomplete method on package controller. This method should be in API (like other autocomplete methods).

Will need to update client code (just forms atm I think).

[thejimmyg]

Child ticket of #954

[dread]

Notes to complete this:

• ckan/logic/action/get.py:'def package_autocomplete' exists and is the one that should be used
• need to ensure all places in the templates and public/scripts use this action directly, not dataset/autocomplete (or its alias package/autocomplete)
• ckan/controllers/package.py:'def autocomplete' should be removed.
• ckan/config/routes.py dataset/autocomplete should be removed.

[icmurray]

autocomplete action has been deprecated in favour of /api/2/util/dataset/autocomplete : ​https://github.com/okfn/ckan/blob/master/ckan/controllers/package.py#L655