Fix import statement in mclag.py (sonic-net#2073)

#### 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:~$
```

Author: Myron Sosyak

config/mclag.py

@@ -85,7 +85,7 @@ def is_ipv4_addr_valid(addr):
 
 
 def check_if_interface_is_valid(db, interface_name):
-    from main import interface_name_is_valid
+    from .main import interface_name_is_valid
     if interface_name_is_valid(db,interface_name) is False:
         ctx.fail("Interface name is invalid. Please enter a valid interface name!!")
 

tests/mclag_test.py

@@ -42,6 +42,9 @@
 MCLAG_INVALID_SESSION_TMOUT_LBOUND  = "2"
 MCLAG_INVALID_SESSION_TMOUT_UBOUND  = "4000"
 
+MCLAG_VALID_PEER_LINK_PORT = "Ethernet0"
+MCLAG_VALID_PEER_LINK_PORTCHANNEL = "PortChannel1000"
+
 MCLAG_INVALID_MCLAG_MEMBER  = "Ethernet4"
 MCLAG_INVALID_PORTCHANNEL1  = "portchannel" 
 MCLAG_INVALID_PORTCHANNEL2  = "PortChannelabcd" 
@@ -142,6 +145,23 @@ def test_add_mclag_with_invalid_peer_mcast_ip(self):
         result = runner.invoke(config.config.commands["mclag"].commands["add"], [MCLAG_DOMAIN_ID, MCLAG_SRC_IP, MCLAG_INVALID_PEER_IP2, MCLAG_PEER_LINK], obj=obj)
         assert result.exit_code != 0, "mclag invalid peer ip mcast test caase with code {}:{} Output:{}".format(type(result.exit_code), result.exit_code, result.output)
 
+    def test_add_mclag_with_valid_peer_link(self):
+        runner = CliRunner()
+        db = Db()
+        obj = {'db':db.cfgdb}
+
+        # add mclag with valid port peer link
+        result = runner.invoke(config.config.commands["mclag"].commands["add"], [MCLAG_DOMAIN_ID, MCLAG_SRC_IP, MCLAG_PEER_IP, MCLAG_VALID_PEER_LINK_PORT], obj=obj)
+        assert result.exit_code == 0, "mclag valid peer link test case with code {}:{} Output:{}".format(type(result.exit_code), result.exit_code, result.output)
+
+        runner = CliRunner()
+        db = Db()
+        obj = {'db':db.cfgdb}
+
+        # add mclag with valid portchannel peer link
+        result = runner.invoke(config.config.commands["mclag"].commands["add"], [MCLAG_DOMAIN_ID, MCLAG_SRC_IP, MCLAG_PEER_IP, MCLAG_VALID_PEER_LINK_PORTCHANNEL], obj=obj)
+        assert result.exit_code == 0, "mclag valid peer link test case with code {}:{} Output:{}".format(type(result.exit_code), result.exit_code, result.output)
+
     def test_add_mclag_with_invalid_peer_link(self):
         runner = CliRunner()
         db = Db()