The depot tool did not consistently distinct libraries from regular
targets when handling binary archives. The binary archive of a regular
target is located at <user>/<bin>/<arch>/<name> whereas a library is
located at <user>/<bin>/<arch>/<api>/<name>.
Fixes#2438
This patch replaces the toy downloader (that merely copied files
locally, for testing) with the mechanics needed to download files from a
www server.
It also changes the use of GPG to use detached signatures.
Issue #2339