Enable custom solution to detect existing packages

With this change, all harvesters that extend the base harvester have the
possibility to use the very useful create_or_update method, but still
define their own way of detecting what package is the existing one.

This is very useful for harvest sources that have no knowledge of the
CKAN internal id, but have another way of finding previous package.

ckanext/harvest/harvesters/base.py

@@ -281,13 +281,8 @@ class HarvesterBase(SingletonPlugin):
                 package_dict['tags'] = tags
 
             # Check if package exists
-            data_dict = {}
-            data_dict['id'] = package_dict['id']
             try:
-                package_show_context = {'model': model, 'session': Session,
-                                        'ignore_auth': True}
-                existing_package_dict = get_action('package_show')(
-                    package_show_context, data_dict)
+                existing_package_dict = self._find_existing_package(package_dict)
 
                 # In case name has been modified when first importing. See issue #101.
                 package_dict['name'] = existing_package_dict['name']
@@ -359,3 +354,10 @@ class HarvesterBase(SingletonPlugin):
             self._save_object_error('%r'%e,harvest_object,'Import')
 
         return None
+
+    def _find_existing_package(self, package_dict):
+        data_dict = {'id': package_dict['id']}
+        package_show_context = {'model': model, 'session': Session,
+                                'ignore_auth': True}
+        return get_action('package_show')(
+            package_show_context, data_dict)