builder: isolate C libraries in an own OCAMLCLIBS

Just moving stuff within Makefile.am, no functional changes.
builder: prepare for different per-protocol download actions

Small refactor of Downloader.download_to to allow different download
actions depending on the protocol of the URI (which is now parsed).

No actual behaviour changes, just mostly code motion.
builder: do a copy when downloading local files

Instead of spawning curl even to "download" file:// URIs, just copy
them.