Cope if the zip buffer is already open, and kill it once we're done.

TODO: leave it open if we weren't the one to open it?
Wired up downloading of source artifacts from Maven.

Also rejiggered blocking rescans. We'll only ever blocking rescan the top-level
project from which you initiate a query. All the depends get rescanned in the
background (immediately, but they don't hold up your query).
Move file() into package.
Added a servlet for serving up m2 and ivy docs.