Use explicit options to control which components get installed

Checking the installed gems is not completely reliable, especially
when it comes to checking their absence. This more explicit mode
allows for more control and good defaults.

lib/generators/solidus_paypal_commerce_platform/install/install_generator.rb

@@ -4,6 +4,8 @@ module SolidusPaypalCommercePlatform
   module Generators
     class InstallGenerator < Rails::Generators::Base
       class_option :migrate, type: :boolean, default: true
+      class_option :backend, type: :boolean, default: true
+      class_option :starter_frontend, type: :boolean, default: true
       class_option :specs, type: :string, enum: %w[all frontend], default: 'frontend'
 
       source_root File.expand_path('templates', __dir__)
@@ -16,7 +18,7 @@ def install_solidus_core_support
       end
 
       def install_solidus_backend_support
-        support_code_for('solidus_backend') do
+        support_code_for(:solidus_backend) do
           append_file(
             'vendor/assets/javascripts/spree/backend/all.js',
             "//= require spree/backend/solidus_paypal_commerce_platform\n"
@@ -31,10 +33,7 @@ def install_solidus_backend_support
       end
 
       def install_solidus_starter_frontend_support
-        solidus_frontend_is_missing = !Bundler.locked_gems.specs.map(&:name).include?('solidus_frontend')
-
-        # We can't do better than this for now, over time we might want to mark things differently
-        support_code_for('solidus_starter_frontend', run_if: solidus_frontend_is_missing) do
+        support_code_for(:solidus_starter_frontend) do
           directory 'app', 'app'
           append_file(
             'app/assets/javascripts/solidus_starter_frontend.js',
@@ -70,12 +69,12 @@ def install_solidus_starter_frontend_support
 
       private
 
-      def support_code_for(gem_name, run_if: Bundler.locked_gems.specs.map(&:name).include?(gem_name), &block)
-        if run_if
-          say_status :install, "[#{engine.engine_name}] #{gem_name} code", :blue
+      def support_code_for(component_name, &block)
+        if options[component_name]
+          say_status :install, "[#{engine.engine_name}] solidus_#{component_name}", :blue
           shell.indent(&block)
         else
-          say_status :skip, "[#{engine.engine_name}] #{gem_name} code", :blue
+          say_status :skip, "[#{engine.engine_name}] solidus_#{component_name}", :blue
         end
       end