Introduce ForkProjectsFinder class

* Will dry up the code to find projects between the ForksController and the API

app/finders/fork_projects_finder.rb

+class ForkProjectsFinder < ProjectsFinder
+  def initialize(project, params: {}, current_user: nil)
+    project_ids = project.forks.includes(:creator).select(:id)
+    super(params: params, current_user: current_user, project_ids_relation: project_ids)
+  end
+end

spec/finders/fork_projects_finder_spec.rb

+require 'spec_helper'
+
+describe ForkProjectsFinder do
+  let(:source_project) { create(:project, :empty_repo) }
+  let(:private_fork) { create(:project, :private, :empty_repo, name: 'A') }
+  let(:internal_fork) { create(:project, :internal, :empty_repo, name: 'B') }
+  let(:public_fork) { create(:project, :public, :empty_repo, name: 'C') }
+
+  let(:non_member) { create(:user) }
+  let(:private_fork_member) { create(:user) }
+
+  before do
+    private_fork.add_developer(private_fork_member)
+
+    source_project.forks << private_fork
+    source_project.forks << internal_fork
+    source_project.forks << public_fork
+  end
+
+  describe '#execute' do
+    let(:finder) { described_class.new(source_project, params: {}, current_user: current_user) }
+
+    subject { finder.execute }
+
+    describe 'without a user' do
+      let(:current_user) { nil }
+
+      it { is_expected.to eq([public_fork]) }
+    end
+
+    describe 'with a user' do
+      let(:current_user) { non_member }
+
+      it { is_expected.to eq([public_fork, internal_fork]) }
+    end
+
+    describe 'with a member' do
+      let(:current_user) { private_fork_member }
+
+      it { is_expected.to eq([public_fork, internal_fork, private_fork]) }
+    end
+  end
+end