libcurl supports TLS/SSL (ie: https:// urls) but not sure about how well that works in Darkplaces.
However, having used curl to download files from a lot of things (deploying files all over the place), the issue might be that the link you're using isn't the final canonical link to the file, but is a redirect. I don't know how well Darkplaces supports redirects (if at all).
If you have a copy of curl, you can try this yourself in a command prompt using the links you've got.
Try:
curl -O
https://your.link.goes/here/whatever.pk3
If that doesn't work, try:
curl -O -J -L
https://your.link.goes/here/whatever.pk3
If the first one doesn't work but the second does, the link is a redirect link.
FWIW: Dropbox uses redirect links when you select 'Download' and get a link that ends in "?dl=0" (which actually makes it open a view window). You can improve that by changing it to "?raw=1"
on the end, but it still does a redirect.
Quote:“To summarize the summary of the summary: people are a problem.” - Douglas Adams