classes/package: fix FILES_INFO serialisation in pkgdata

classes/package: fix FILES_INFO serialisation in pkgdata

The FILES_INFO entry in each pkgdata file stores the list of files for
each package. Make the following improvements to how this is stored:
* Store paths as they would be seen on the target rather than
  erroneously including the full path to PKGDEST (which is specific to
  the build host the package was built on)
* For simplicity when loading the data, store complete paths for each
  entry instead of trying to break off the first part and use it as the
  dict key
* Record sizes for each file (as needed by Toaster)
* Serialise the value explicitly using json rather than just passing it
  through str().

Fixes [YOCTO #5443].

(From OE-Core rev: ca86603607a69a17cc5540d69de0e242b33382d3)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>