read_wim_lookup_table(): Allow multiple "subpacks" per packed run

read_wim_lookup_table():  Allow multiple "subpacks" per packed run

Apparently WIMGAPI does, in fact, support multiple packed resources per
WIM.  It creates them when exporting, and probably when appending too.
However, the format of the resulting lookup table is a little different
than I had expected, since it seems to be required that all the packed
resource metadata be combined into a single, mostly unordered run of
entries with the 0x10 flag set.  This commit updates the read-side code
with this new assumption.