Normally, the APIs listed in 'used_apis' files do not carry a version
but implictly refer to the current version. This patch allows for
specifiying an concrete version. This is a useful feature in the context
of the Goa tool.
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
