[dhcp_relay] Build dhcprelay from dhcp6relay folder when available (#23785)

Why I did it
Moving DHCP v6 Relay code to dhcp6relay folder in sonic-dhcp-relay repo.
For compilation to succeed when the code is moved , these changes are raised to compile if dhcp6relay folder is present.
Otherwise it will compile as before.

Corresponding sonic-dhcp-relay PR: sonic-net/sonic-dhcp-relay#72

How I did it
Added a check to verify if dhcprelay/dhcp6relay exists , then it will compile from that folder.

How to verify it
Default compilation
Move DHCP v6 related code to dhcprelay/dhcp6relay folder
Both above cases code should compile.

Author: cshivashgit

rules/dhcprelay.mk

@@ -5,7 +5,11 @@ SONIC_DHCPRELAY_PKG_NAME = dhcp6relay
 
 SONIC_DHCPRELAY = sonic-$(SONIC_DHCPRELAY_PKG_NAME)_$(SONIC_DHCPRELAY_VERSION)_$(CONFIGURED_ARCH).deb
 $(SONIC_DHCPRELAY)_DEPENDS = $(LIBSWSSCOMMON) $(LIBSWSSCOMMON_DEV)
-$(SONIC_DHCPRELAY)_SRC_PATH = $(SRC_PATH)/dhcprelay
+ifneq ("$(wildcard $(SRC_PATH)/dhcprelay/dhcp6relay)","")
+  $(SONIC_DHCPRELAY)_SRC_PATH = $(SRC_PATH)/dhcprelay/dhcp6relay
+else
+  $(SONIC_DHCPRELAY)_SRC_PATH = $(SRC_PATH)/dhcprelay
+endif
 SONIC_DPKG_DEBS += $(SONIC_DHCPRELAY)
 
 SONIC_DHCPRELAY_DBG = sonic-$(SONIC_DHCPRELAY_PKG_NAME)-dbgsym_$(SONIC_DHCPRELAY_VERSION)_$(CONFIGURED_ARCH).deb