Move Codegen files into codegen/ directory

Change-Id: I0eec4ee0d27ab39afb971328efadf31807304011

internal/ceres/CMakeLists.txt

diff --git a/internal/ceres/CMakeLists.txt b/internal/ceres/CMakeLists.txt
index 738b431..9e64b89 100644
--- a/internal/ceres/CMakeLists.txt
+++ b/internal/ceres/CMakeLists.txt
@@ -150,10 +150,10 @@
 
 if(CODE_GENERATION)
   set(CERES_CODEGEN_SRC
-      code_generator.cc
-      expression.cc
-      expression_graph.cc
-      expression_ref.cc
+      codegen/code_generator.cc
+      codegen/expression.cc
+      codegen/expression_graph.cc
+      codegen/expression_ref.cc
       codegen/optimize_expression_graph.cc
       codegen/eliminate_nops.cc
   )

internal/ceres/codegen/code_generator.cc

diff --git a/internal/ceres/code_generator.cc b/internal/ceres/codegen/code_generator.cc
similarity index 100%
rename from internal/ceres/code_generator.cc
rename to internal/ceres/codegen/code_generator.cc

internal/ceres/codegen/expression.cc

diff --git a/internal/ceres/expression.cc b/internal/ceres/codegen/expression.cc
similarity index 100%
rename from internal/ceres/expression.cc
rename to internal/ceres/codegen/expression.cc

internal/ceres/codegen/expression_graph.cc

diff --git a/internal/ceres/expression_graph.cc b/internal/ceres/codegen/expression_graph.cc
similarity index 100%
rename from internal/ceres/expression_graph.cc
rename to internal/ceres/codegen/expression_graph.cc

internal/ceres/codegen/expression_ref.cc

diff --git a/internal/ceres/expression_ref.cc b/internal/ceres/codegen/expression_ref.cc
similarity index 100%
rename from internal/ceres/expression_ref.cc
rename to internal/ceres/codegen/expression_ref.cc