Rename site-publisher package to file-publisher

While developing the PDF assembler extension, I came to the realization that the site publisher is, in fact, a general purpose file publisher. Its one and only function is to publish virtual files (as returned by getFiles() of each entry in the catalogs argument) to one or more destinations (as defined in the playbook argument). There's no need to get "site" involved in the terminology here. The main export should be publishFiles not publishSite. In light of that, I think we should rename the site-publisher package to file-publisher. To prevent breaking existing workflows, we can continue to publish site-publisher as an alias (since the two packages would require the same exact code).

As a follow-up, I'd also like to see the publishFiles function accept a single catalog in addition to an array of catalogs. That just makes the function easier to reuse.