makebuildserver: use original names for downloaded SDK components

makebuildserver: use original names for downloaded SDK components

It will make it a lot easier to manage the cache if we use the original
file names, which often include the file version.  This also changes the
download process to be resumable if there is a partial file in the cache,
and switches from calling wget on the command line to using the python libs
'requests' and 'clint' to provide a similar experience.  While its not so
important for this particular bit of code to use those libraries, I think
those two will allow us to provide a better user experience throughout the
whole of fdroidserver.

In this case, it is already doing special tricks fetching the file size
from the server before trying to download it.  I suppose this code could
instead check if the file exists, and if so, check the hash sum.  I think
that would be slower for most people since checking the hash on large files
takes a noticeable about of time, while a HTTP HEAD request is pretty tiny.