add group finder spec and logic

lib/gitlab/import_export/group_project_finder.rb

+module Gitlab
+  module ImportExport
+    class GroupProjectFinder
+      def self.find(*args)
+        new(*args).find
+      end
+
+      def initialize(klass, attributes)
+        @klass = klass
+        @attributes = attributes
+        @group_id = @attributes['group_id']
+        @project_id = @attributes['project_id']
+      end
+
+      def find
+        @klass.where(where_clause)
+      end
+
+      private
+
+      def where_clause
+        @attributes.except('group_id', 'project_id').map do |key, value|
+          table[key].eq(value).and(table[:group_id].eq(@group_id))
+            .or(table[key].eq(value).and(table[:project_id].eq(@project_id)))
+        end.reduce(:or)
+      end
+
+      def table
+        @table ||= @klass.arel_table
+      end
+    end
+  end
+end

spec/lib/gitlab/import_export/group_project_finder_spec.rb

+require 'spec_helper'
+
+describe Gitlab::ImportExport::GroupProjectFinder do
+  let(:project) do
+    create(:project,
+           :builds_disabled,
+           :issues_disabled,
+           name: 'project',
+           path: 'project',
+           group: create(:group))
+  end
+
+  it 'finds the right group label' do
+    group_label = create(:group_label, 'name': 'group label', 'group': project.group)
+
+    expect(described_class.find(Label,
+                                title: 'group label',
+                                'project_id': project.id,
+                                'group_id': project.group.id)).to eq([group_label])
+  end
+end