split_and_strip_files: regroup hardlinks to make build deterministic

Reverted 7c0fd561

Created separate group of hardlinks for the files inside
the same package. This should prevent stripped files to be
populated outside of package directories.

This turns out not to be straightforward and has overlap with the
other hardlink handling code in this area. The code is condensed
into a more concise and documented form.

[Original patch from Ed with tweaks from RP]

[YOCTO #7586]

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>