[dhcp_relay]Fix the clear dhcp6relay_counters CLI (#13148)

Avoid traceback on sonic-clear command

sonic-clear dhcp6relay_counters 
Traceback (most recent call last):
  File "/usr/local/bin/sonic-clear", line 8, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.9/dist-packages/clear/plugins/dhcp-relay.py", line 19, in dhcp6relay_clear_counters
    counter = DHCPv6_Counter()
NameError: name 'DHCPv6_Counter' is not defined

- How I did it
Corrected the way to import using importlib

- How to verify it
Tested the sonic-clear command and verified no traceback is seen

dockers/docker-dhcp-relay/cli/clear/plugins/clear_dhcp6relay_counter.py

@@ -1,7 +1,7 @@
 import sys
 import click
 import importlib
-importlib.import_module('show.plugins.dhcp-relay')
+dhcp6_relay = importlib.import_module('show.plugins.dhcp-relay')
 
 import utilities_common.cli as clicommon
 
@@ -16,7 +16,7 @@ def dhcp6relay_clear():
 def dhcp6relay_clear_counters(interface):
     """ Clear dhcp6relay message counts """
 
-    counter = DHCPv6_Counter()
+    counter = dhcp6_relay.DHCPv6_Counter()
     counter_intf = counter.get_interface()
 
     if interface: