Adjust constant evaluation for single running seeds

This fixes the case where we'd like to filter
a single seed file, e.g.:

bundle exec rake db:seed_fu FILTER=<seed>

db/fixtures/development/02_users.rb

@@ -5,7 +5,6 @@ class Gitlab::Seeder::Users
 
   RANDOM_USERS_COUNT = 20
   MASS_USERS_COUNT = ENV['CI'] ? 10 : 1_000_000
-  MASS_INSERT_USERNAME_START = 'mass_insert_user_'
 
   attr_reader :opts
 
@@ -29,7 +28,7 @@ class Gitlab::Seeder::Users
       ActiveRecord::Base.connection.execute <<~SQL
         INSERT INTO users (username, name, email, confirmed_at, projects_limit, encrypted_password)
         SELECT
-          '#{MASS_INSERT_USERNAME_START}' || seq,
+          '#{Gitlab::Seeder::MASS_INSERT_USER_START}' || seq,
           'Seed user ' || seq,
           'seed_user' || seq || '@example.com',
           to_timestamp(seq),

db/fixtures/development/03_project.rb

@@ -52,7 +52,6 @@ class Gitlab::Seeder::Projects
     internal: 1, # 1m projects +
     public: 1 # 1m projects = 5m total
   }
-  MASS_INSERT_NAME_START = 'mass_insert_project_'
 
   def seed!
     Sidekiq::Testing.inline! do
@@ -167,7 +166,7 @@ class Gitlab::Seeder::Projects
         INSERT INTO projects (name, path, creator_id, namespace_id, visibility_level, created_at, updated_at)
         SELECT
           'Seed project ' || seq || ' ' || ('{#{visibility_per_user}}'::text[])[seq] AS project_name,
-          'mass_insert_project_' || ('{#{visibility_per_user}}'::text[])[seq] || '_' || seq AS project_path,
+          '#{Gitlab::Seeder::MASS_INSERT_PROJECT_START}' || ('{#{visibility_per_user}}'::text[])[seq] || '_' || seq AS project_path,
           u.id AS user_id,
           n.id AS namespace_id,
           ('{#{visibility_level_per_user}}'::int[])[seq] AS visibility_level,

lib/gitlab/seeder.rb

@@ -16,6 +16,8 @@ module Gitlab
   class Seeder
     extend ActionView::Helpers::NumberHelper
 
+    MASS_INSERT_PROJECT_START = 'mass_insert_project_'
+    MASS_INSERT_USER_START = 'mass_insert_user_'
     ESTIMATED_INSERT_PER_MINUTE = 2_000_000
     MASS_INSERT_ENV = 'MASS_INSERT'
 
@@ -24,7 +26,7 @@ module Gitlab
 
       included do
         scope :not_mass_generated, -> do
-          where.not("path LIKE '#{Gitlab::Seeder::Projects::MASS_INSERT_NAME_START}%'")
+          where.not("path LIKE '#{MASS_INSERT_PROJECT_START}%'")
         end
       end
     end
@@ -34,7 +36,7 @@ module Gitlab
 
       included do
         scope :not_mass_generated, -> do
-          where.not("username LIKE '#{Gitlab::Seeder::Users::MASS_INSERT_USERNAME_START}%'")
+          where.not("username LIKE '#{MASS_INSERT_USER_START}%'")
         end
       end
     end