Refactoring the core security code.

As opposed to a flat directory, we now have the following structure:

- security
  -context
  - credentials
    - composite
    - fake
    - google_default
    - iam
    - jwt
    - oauth2
    - plugin
    - ssl
  - transport
  - util

We have not refactored the test code yet but this PR is already large
enough...
diff --git a/src/core/ext/transport/chttp2/server/secure/server_secure_chttp2.c b/src/core/ext/transport/chttp2/server/secure/server_secure_chttp2.c
index 698b2be..2c9f013 100644
--- a/src/core/ext/transport/chttp2/server/secure/server_secure_chttp2.c
+++ b/src/core/ext/transport/chttp2/server/secure/server_secure_chttp2.c
@@ -45,10 +45,10 @@
 #include "src/core/lib/iomgr/endpoint.h"
 #include "src/core/lib/iomgr/resolve_address.h"
 #include "src/core/lib/iomgr/tcp_server.h"
-#include "src/core/lib/security/auth_filters.h"
-#include "src/core/lib/security/credentials.h"
-#include "src/core/lib/security/security_connector.h"
-#include "src/core/lib/security/security_context.h"
+#include "src/core/lib/security/context/security_context.h"
+#include "src/core/lib/security/credentials/credentials.h"
+#include "src/core/lib/security/transport/auth_filters.h"
+#include "src/core/lib/security/transport/security_connector.h"
 #include "src/core/lib/surface/api_trace.h"
 #include "src/core/lib/surface/server.h"