Cleanup includes. (#1795)

This Cl cleans up the include paths to be relative to the top level
directory. Various include-what-you-use fixes have been added.
diff --git a/source/enum_string_mapping.cpp b/source/enum_string_mapping.cpp
index 20bbdfc..32361a0 100644
--- a/source/enum_string_mapping.cpp
+++ b/source/enum_string_mapping.cpp
@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "enum_string_mapping.h"
+#include "source/enum_string_mapping.h"
 
 #include <algorithm>
 #include <cassert>
@@ -20,7 +20,7 @@
 #include <string>
 #include <unordered_map>
 
-#include "extensions.h"
+#include "source/extensions.h"
 
 namespace spvtools {