Implement new fetchers for package-build.el

[wasamasa]

package-build.el supports a good range of useful fetchers, but if I see correctly not downloading a single file, multiple files (or a directory) or archives yet. Is there a reason why not?

[purcell]

Yes, there's a reason: it's not possible to reliably assign versions to files downloaded from arbitrary locations. When we download source files, we need to be able to (reproducibly) figure out when they last changed, so this limits us to upstream locations which provide us with history metadata.