[dhcp_relay]Fix the clear dhcp6relay_counters CLI

[dgsudharsan]

Why I did it

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

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211

Description for the changelog

Ensure to add label/tag for the feature raised. example - PR#2174 under sonic-utilities repo. where, Generic Config and Update feature has been labelled as GCU.

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)