Refactor GN source expansions.

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 30 matching lines @@
         'config_values_extractors.cc',
         'config_values_extractors.h',
         'config_values_generator.cc',
         'config_values_generator.h',
         'copy_target_generator.cc',
         'copy_target_generator.h',
         'err.cc',
         'err.h',
         'escape.cc',
         'escape.h',
-        'file_template.cc',
-        'file_template.h',
         'filesystem_utils.cc',
         'filesystem_utils.h',
         'functions_target.cc',
         'functions.cc',
         'functions.h',
         'function_exec_script.cc',
         'function_foreach.cc',
         'function_get_label_info.cc',
         'function_get_path_info.cc',
         'function_get_target_outputs.cc',
@@ skipping 66 matching lines @@
         'setup.cc',
         'setup.h',
         'source_dir.cc',
         'source_dir.h',
         'source_file.cc',
         'source_file.h',
         'standard_out.cc',
         'standard_out.h',
         'string_utils.cc',
         'string_utils.h',
+        'substitution_list.cc',
+        'substitution_list.h',
+        'substitution_pattern.cc',
+        'substitution_pattern.h',
+        'substitution_type.cc',
+        'substitution_type.h',
+        'substitution_writer.cc',
+        'substitution_writer.h',
         'target.cc',
         'target.h',
         'target_generator.cc',
         'target_generator.h',
         'template.cc',
         'template.h',
         'token.cc',
         'token.h',
         'tokenizer.cc',
         'tokenizer.h',
@@ skipping 25 matching lines @@
     },
     {
       'target_name': 'gn_unittests',
       'type': '<(gtest_target_type)',
       'sources': [
         'builder_unittest.cc',
         'c_include_iterator_unittest.cc',
         'config_values_extractors_unittest.cc',
         'escape_unittest.cc',
         'filesystem_utils_unittest.cc',
-        'file_template_unittest.cc',
         'function_foreach_unittest.cc',
         'function_get_label_info_unittest.cc',
         'function_get_path_info_unittest.cc',
         'function_get_target_outputs_unittest.cc',
         'function_rebase_path_unittest.cc',
         'function_write_file_unittest.cc',
         'functions_target_unittest.cc',
         'functions_unittest.cc',
         'header_checker_unittest.cc',
         'input_conversion_unittest.cc',
         'label_unittest.cc',
         'loader_unittest.cc',
         'ninja_action_target_writer_unittest.cc',
         'ninja_binary_target_writer_unittest.cc',
         'ninja_copy_target_writer_unittest.cc',
         'ninja_helper_unittest.cc',
         'ninja_target_writer_unittest.cc',
         'operators_unittest.cc',
         'parse_tree_unittest.cc',
         'parser_unittest.cc',
         'path_output_unittest.cc',
         'pattern_unittest.cc',
         'run_all_unittests.cc',
         'scope_per_file_provider_unittest.cc',
         'scope_unittest.cc',
         'source_dir_unittest.cc',
         'string_utils_unittest.cc',
+        'substitution_pattern_unittest.cc',
+        'substitution_writer_unittest.cc',
         'target_generator_unittest.cc',
         'target_unittest.cc',
         'test_with_scope.cc',
         'test_with_scope.h',
         'tokenizer_unittest.cc',
         'value_unittest.cc',
         'visibility_unittest.cc',
       ],
       'dependencies': [
         'gn_lib',
         '../../base/base.gyp:test_support_base',
         '../../testing/gtest.gyp:gtest',
       ],
     },
     {
       'target_name': 'generate_test_gn_data',
       'type': 'executable',
       'sources': [
         'generate_test_gn_data.cc',
       ],
       'dependencies': [
         '../../base/base.gyp:base',
       ],
     }
   ],
 }