This package contains only one file.
uscan finds the correct .js file, not an archive.
The .orig file can actually be made by running:
 export DEB_VERSION_UPSTREAM=$(dpkg-parsechangelog --show-field version | sed -e 's/-[^-]*$//')
 wget https://code.jquery.com/jquery-migrate-${DEB_VERSION_UPSTREAM}.js -O jquery-migrate-${DEB_VERSION_UPSTREAM}.js
 tar czf ../jquery-migrate-1_${DEB_VERSION_UPSTREAM}.orig.tar.gz jquery-migrate-${DEB_VERSION_UPSTREAM}.js

If there ever is a new 1.* release, the source should be taken from
https://github.com/jquery/jquery-migrate
