Hide libcurl internally instead of wrapping

[Hind-M]

Fix #159

TODO:

Two points still need to be handled (maybe after completing powerloader integration in mamba? - mostly for the first one):

• ssl_backend_t enum conversion to use outside powerloader.
• progress_callback_t = std::function<int(curl_off_t, curl_off_t)> in download_target.hpp is apparently used in mamba with std::size_t instead of curl_off_t. So I don't see why curl_off_t is used here.
  If there is no specific reason, it should directly use std::size_t.