tools/pyang: exclude common YANG models from import to avoid duplicates

[dudu579]

sonic-mgmt-common extension name:

extension custom-validation {
		description
			"Extension for custom validation. 
			 Platform specific validation can be implemented using custom validation.";
		argument "handler"; 
	}

sonic-yang-models auto-generated extension name:

extension custom-validation-cvl {
        description "Extension for registering cvl based custom validation handler.";
        argument "handler";
    }

mgmt-framework imports both, this conflict happens.

Related issue:

• sonic-extension.yang conflicted with auto-generated sonic-extension.yang in sonic-yang-models #217

[linux-foundation-easycla]

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• ✅ login: dudu579 / name: Jichen Dou (37edd00)

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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