Add scope to exclude project namespaces

There will be multiple places where we will have to exclude project
namespaces when getting Namespace so let's add a scope for it.

app/models/namespace.rb

@@ -116,6 +116,9 @@ class Namespace < ApplicationRecord
   # TODO: change to `type: Namespaces::UserNamespace.sti_name` when
   #       working on issue https://gitlab.com/gitlab-org/gitlab/-/issues/341070
   scope :user_namespaces, -> { where(type: [nil, Namespaces::UserNamespace.sti_name]) }
+  # TODO: this can be simplified with `type != 'Project'`  when working on issue
+  #       https://gitlab.com/gitlab-org/gitlab/-/issues/341070
+  scope :without_project_namespaces, -> { where("type IS DISTINCT FROM ?", Namespaces::ProjectNamespace.sti_name) }
   scope :sort_by_type, -> { order(Gitlab::Database.nulls_first_order(:type)) }
   scope :include_route, -> { includes(:route) }
   scope :by_parent, -> (parent) { where(parent_id: parent) }

spec/models/namespace_spec.rb

@@ -299,6 +299,15 @@ RSpec.describe Namespace do
         expect(described_class.sorted_by_similarity_and_parent_id_desc('Namespace')).to eq([namespace2, namespace1, namespace2sub, namespace1sub, namespace])
       end
     end
+
+    describe '.without_project_namespaces' do
+      let_it_be(:user_namespace) { create(:user_namespace) }
+      let_it_be(:project_namespace) { create(:project_namespace) }
+
+      it 'excludes project namespaces' do
+        expect(described_class.without_project_namespaces).to match_array([namespace, namespace1, namespace2, namespace1sub, namespace2sub, user_namespace, project_namespace.parent])
+      end
+    end
   end
 
   describe 'delegate' do