Normalize collection content artifact filename when generating site.
We allowed specification of a "content base name" for collections (e.g. index), and allow several of them to be indicated. such as foo.html and bar.html, the same names will be used during mummification. But some deployment targets such as AWS S3 don't allow multiple "index documents" to be specified, so we should normalize the content name to the first one given when mummifying the site.
Don't forget to remove the AWS S3 Website deployment warning for multiple content base names, now that we've normalized to a single content artifact name.
With GUISE-125, now that we explicitly determine target paths in a relative way at each level of the hierarchy by the parent artifact mummifier, this should be pretty easy to implement.