Move toolchain to a subdir

src/dds/build.cpp

 #include <dds/logging.hpp>
 #include <dds/proc.hpp>
 #include <dds/source.hpp>
-#include <dds/toolchain.hpp>
+#include <dds/toolchain/toolchain.hpp>
 #include <dds/compdb.hpp>
 #include <dds/usage_reqs.hpp>
 #include <dds/util/algo.hpp>

src/dds/build/archive.hpp

 #pragma once
 
-#include <dds/toolchain.hpp>
+#include <dds/toolchain/toolchain.hpp>
 #include <dds/util/fs.hpp>
 
 #include <vector>

src/dds/build/params.hpp

 #pragma once
 
-#include <dds/toolchain.hpp>
+#include <dds/toolchain/toolchain.hpp>
 #include <dds/util/fs.hpp>
 
 namespace dds {

src/dds/build/plan/base.hpp

 #pragma once
 
-#include <dds/toolchain.hpp>
+#include <dds/toolchain/toolchain.hpp>
 #include <dds/util/fs.hpp>
 
 namespace dds {

src/dds/toolchain.test.cpp → src/dds/toolchain/toolchain.test.cpp

-#include <dds/toolchain.hpp>
+#include <dds/toolchain/toolchain.hpp>
 
 #include <dds/util.test.hpp>