Rename "PacketTransportInterface" to "PacketTransportInternal".

This is the naming scheme we've been using for internal interfaces.

Also, this CL will introduce a PacketTransportInterface in the webrtc namespace,
which would get too easily confused with the rtc:: one:
https://codereview.webrtc.org/2675173003/

BUG=None

Review-Url: https://codereview.webrtc.org/2679103006
Cr-Commit-Position: refs/heads/master@{#16539}
diff --git a/webrtc/p2p/BUILD.gn b/webrtc/p2p/BUILD.gn
index 9967b12..07fcdf4 100644
--- a/webrtc/p2p/BUILD.gn
+++ b/webrtc/p2p/BUILD.gn
@@ -39,6 +39,7 @@
     "base/p2ptransportchannel.h",
     "base/packetsocketfactory.h",
     "base/packettransportinterface.h",
+    "base/packettransportinternal.h",
     "base/port.cc",
     "base/port.h",
     "base/portallocator.cc",