debuginfod: Add a flag to limit scan depth

debuginfod: Add a flag to limit scan depth

As of now the scanner may consume a lot of time, descending deep into
irrelevant directories where no build artifacts are known to be stored.

The change introduces --max-depth flag that has semantics similar to
-maxdepth in GNU find.

Signed-off-by: Roman Bolshakov <rbolshakov@ddn.com>