Trying to follow along the example in the Exoscale article about creating custom templates, you get the error about exoscale-import not found, when you are trying to validate the packer template.
Make sure that you use a current packer installation as the exoscale-import was not supported in the older versions.
kosmas:terraform (add_vault *%)$ packer --version 1.0.4
So after installing the latest version the above error goes away
kosmas:terraform (add_vault *%)$ packer --version 1.6.1