sage-download-file could use the metalink or torrent file for upstream spkgs

[haraldschilly]

Since we have switched to Git and have this spkg/upsteam directory, it is also indexed it like the other mirrored files. This means, there is a metalink and and torrent file (webseeds only!) for each file. We could use aria2c as a nice command-line utility to download the file from all the mirrors with included verification.

E.g.
/spkg/upstream/ipython/ipython-0.13.2.tar.bz2
has corresponding files:
/spkg/upstream/ipython/meta/ipython-0.13.2.tar.bz2.metalink|.torrent

The logic should be like this:

if aria2c exists:
 if corresponding metalink file exists:
  aria2c ...
else:
 continue with curl, wget, ...

The only complication is, that it doesn't download to stdout. That must be wrapped somehow with a temporary file.

Component: misc

Issue created by migration from https://trac.sagemath.org/ticket/15544