Refactor GN source expansions.

tools/gn/BUILD.gn

Index: tools/gn/BUILD.gn
diff --git a/tools/gn/BUILD.gn b/tools/gn/BUILD.gn
index 66bbf2f97fced44b3c3f4be8be4efc5c3cec368c..6fbe8aa0f6638a1e629f28d57f92c367c6170f8d 100644
--- a/tools/gn/BUILD.gn
+++ b/tools/gn/BUILD.gn
@@ -44,8 +44,6 @@ static_library("gn_lib") {
     "err.h",
     "escape.cc",
     "escape.h",
-    "file_template.cc",
-    "file_template.h",
     "filesystem_utils.cc",
     "filesystem_utils.h",
     "functions.cc",
@@ -132,6 +130,14 @@ static_library("gn_lib") {
     "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",
@@ -180,7 +186,6 @@ test("gn_unittests") {
     "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",
@@ -208,6 +213,8 @@ test("gn_unittests") {
     "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",
     "template_unittest.cc",