These warnings are triggered by requests either using byte offsets or
reading a number of bytes that is not a multiple of the block size as
well as by components using the plugin with a different block size than
the backend block session provides.
Fixes#1964.
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082