lib/bb/cache.py: invalidate cache when file checksum entry no longer exists

Go through the cached list of file checksums and check if any of the
files no longer exist; if any are missing then invalidate the cached
recipe, which will force it to be reparsed and thus force the list of
files to be collected again. This prevents a warning when moving a file
to a different location that is still picked up by the recipe, e.g.
moving a file from a "files" subdirectory to one named with the recipe
name (${BPN}).

Fixes [YOCTO #4474].

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