x/ref: Restructure security packages

Here are the renamings:

security               -> lib/security
security/audit         -> lib/security/audit
security/flag          -> lib/security/securityflag
security/serialization -> lib/security/serialization

security/agent                -> services/agent
security/agent/agentd         -> services/agent/agentd
security/agent/keymgr         -> services/agent/keymgr
security/agent/server         -> services/agent/server
security/agent/cache          -> services/agent/internal/cache
security/agent/lru            -> services/agent/internal/lru
security/agent/pingpong       -> services/agent/internal/pingpong
security/agent/test_principal -> services/agent/internal/test_principal

Note that the restructuring is not done, even after this CL is
submitted.  In particular, services/agent is supposed to contain
the *.vdl files, and the client and server libraries are supposed
to live in services/agent/agentlib, per our conventions.  Also
lib/unixfd can also be moved under services/agent/internal.  That
will be fixed in a subsequent CL.

MultiPart: 1/3
Change-Id: Ie9b14083d133a661f0d3a6c33a3ae05e3bac3cd2
diff --git a/test/modules/shell.go b/test/modules/shell.go
index d72010e..729c360 100644
--- a/test/modules/shell.go
+++ b/test/modules/shell.go
@@ -158,8 +158,8 @@
 	"v.io/v23/security"
 	"v.io/x/ref/envvar"
 	"v.io/x/ref/lib/exec"
-	"v.io/x/ref/security/agent"
-	"v.io/x/ref/security/agent/keymgr"
+	"v.io/x/ref/services/agent"
+	"v.io/x/ref/services/agent/keymgr"
 	"v.io/x/ref/test/expect"
 )