Detangle p2p/connection.cc and port.cc

This CL does:
- Run IWYU on the relevant elements
- Make connection depend on port_interface, not port
- Make port_allocator depend only on port
- Move some constants from port.h into p2p_constants

This allows a dependency graph without ugly groups.

Bug: webrtc:15796
Change-Id: I0ff0e14eacdfe3b230a8d84902a78eb062d6c8af
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/336320
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41618}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index cb23ea3..08e33ca 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -334,7 +334,7 @@
     ":sequence_checker",
     ":turn_customizer",
     "../call:rtp_interfaces",
-    "../p2p:connection_and_port",
+    "../p2p:connection",
     "../p2p:rtc_p2p",
     "../pc:media_factory",
     "../rtc_base:copy_on_write_buffer",
@@ -936,7 +936,7 @@
     ":make_ref_counted",
     ":packet_socket_factory",
     ":scoped_refptr",
-    "../p2p:connection_and_port",
+    "../p2p:connection",
     "../p2p:ice_transport_internal",
     "../p2p:p2p_constants",
     "../p2p:p2p_transport_channel",