[config][mclag] Fix import statement in mclag.py

[msosyak]

What I did

Fixed import error
Added unit test for that case

How I did it

Used relative import to import file from the current package.

How to verify it

Try to configure mclag with Port as peer interface

sudo config mclag add 1 192.168.3.1 192.168.3.2 Ethernet12

Previous command output (if the output of a command-line utility has changed)

admin@sonic:~$ sudo config mclag add 1 192.168.3.1 192.168.3.2 Ethernet12
Traceback (most recent call last):
  File "/usr/local/bin/config", line 8, in <module>
    sys.exit(config())
  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 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/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/local/lib/python3.9/dist-packages/config/mclag.py", line 140, in add_mclag_domain
    if (peer_ifname.startswith("Ethernet") is True) and (check_if_interface_is_valid(db, peer_ifname) is False):
  File "/usr/local/lib/python3.9/dist-packages/config/mclag.py", line 89, in check_if_interface_is_valid
    from main import interface_name_is_valid
ModuleNotFoundError: No module named 'main'

New command output (if the output of a command-line utility has changed)

admin@sonic:~$ sudo config mclag add 1 192.168.3.1 192.168.3.2 Ethernet12
admin@sonic:~$

[msosyak]

@qiluo-msft @xumia Please review

[qiluo-msft]

We recently fixed the build on master branch. Could you rebase to master or merge master?

[msosyak]

@qiluo-msft CI passed, please help to merge.

[msosyak]

@qiluo-msft @arlakshm Could we merge this?