Use the actual extension name rather than the module name

[derekbekoe]

fixes knack regression

Fixes the following:

Actual:

$ az image copy -h
This command is from the following extension: azext_imagecopy

Expected:

$ az image copy -h
This command is from the following extension: azure-cli-image-copy-extension

[azuresdkci]

View a preview at https://prompt.ws/r/Azure/azure-cli/5126
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[tjprescott]

And thank you for creating solutions not issues! 😁