update imports to import from corehttp when generating with --unbranding=true

[iscai-msft]

We want to update all of the imports for an unbranded SDK to import from corehttp instead of from azure.core. The cleanest way to do this will be just updating the import type in autorest.codegen.models.imports.py. Here, if we are unbranded, and we see an import for azure.core, we just change it to an import from corehttp. The names (besides AzureKeyCredential) are all the same, so this should automatically work once we update the imports serializer

I believe the only caveat to this is we have changed the name of AzureKeyCredential to ServiceKeyCredential in gencore. Therefore, we should update our autorest.codegen.models.AzureKeyCredentialType and autorest.codegen.models.AzureKeyCredentialPolicyType to be called autorest.codegen.models.KeyCredentialType and autorest.codegen.models.KeyCredentialPolicyType, and have a property self.name on it, which will be ServiceKeyCredential if --unbranding==true, otherwise, ServiceKeyCredential.