Problem: publication's distributions are full URLs

Solution: use DetailRelatedField instead of HyperlinkedRelatedField

The serializer was using the wrong type of field for serializing the Publication's distrubtions field.

fixes: #4590
https://pulp.plan.io/issues/4590

pulpcore/app/serializers/publication.py

@@ -27,12 +27,13 @@ class PublicationSerializer(ModelSerializer):
         help_text=_('The publisher that created this publication.'),
         queryset=models.Publisher.objects.all()
     )
-    _distributions = serializers.HyperlinkedRelatedField(
+    _distributions = DetailRelatedField(
         help_text=_('This publication is currently being served as'
                     'defined by these distributions.'),
         many=True,
         read_only=True,
         view_name='distributions-detail',
+        queryset=models.BaseDistribution.objects.all()
     )
     repository_version = NestedRelatedField(
         view_name='versions-detail',