This change keeps the version-controlled 'pubkey' and 'download' files
separate from files generated via depot/create or downloaded via
depot/download. So one can remove the entire depot/ directory without
interfering with git.
Furthermore, depot keys can now be hosted in supplemental repositories
independent from Genode's main repository.
Fixes#4364
This patch changes the depot layout such that each archive is
represented as a directory that contains the versions of the archive as
subdirectories.
Issue #2610
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
