[MCPE-19966] The game became more finicky about the structure of .mcworld / .mcpack files

MCPE no longer successfully imports .mcworld / .mcpack files with one level of superfluous directory. I believe this is a regression.

The attached file "ZombieNetherPortalSpawn.mcworld" has the following structure and can not be imported successfully:

% unzip -l ZombieNetherPortalSpawn.mcworld
Archive:  ZombieNetherPortalSpawn.mcworld
  Length     Date   Time    Name
 --------    ----   ----    ----
        0  02-09-17 09:15   H5MAALzHCAA=/level.dat_old
        0  02-09-17 09:15   H5MAALzHCAA=/level.dat
        0  02-09-17 09:15   H5MAALzHCAA=/db/LOCK
        0  02-09-17 09:15   H5MAALzHCAA=/db/000009.ldb
        0  02-09-17 09:15   H5MAALzHCAA=/db/000012.ldb
        0  02-09-17 09:15   H5MAALzHCAA=/db/CURRENT
        0  02-09-17 09:15   H5MAALzHCAA=/db/000005.ldb
        0  02-09-17 09:15   H5MAALzHCAA=/db/000007.ldb
        0  02-09-17 09:15   H5MAALzHCAA=/db/000015.ldb
        0  02-09-17 09:15   H5MAALzHCAA=/db/000016.log
        0  02-09-17 09:15   H5MAALzHCAA=/db/MANIFEST-000014
        0  02-09-17 09:15   H5MAALzHCAA=/levelname.txt
        0  02-09-17 09:15   H5MAALzHCAA=/world_resource_packs.json
        0  02-09-17 09:15   H5MAALzHCAA=/world_behavior_packs.json
 --------                   -------
        0                   14 files

On the other hand, "ZombieNetherPortalSpawn.modified.mcworld" has the following structure and just works:

% unzip -l ZombieNetherPortalSpawn.modified.mcworld
Archive:  ZombieNetherPortalSpawn.modified.mcworld
  Length     Date   Time    Name
 --------    ----   ----    ----
        0  02-10-17 20:53   db/
        0  02-09-17 09:15   db/LOCK
    21653  02-09-17 09:15   db/000009.ldb
     1222  02-09-17 09:15   db/000012.ldb
       16  02-09-17 09:15   db/CURRENT
   121406  02-09-17 09:15   db/000005.ldb
   135167  02-09-17 09:15   db/000007.ldb
     2943  02-09-17 09:15   db/000015.ldb
     2736  02-09-17 09:15   db/000016.log
      279  02-09-17 09:15   db/MANIFEST-000014
      796  02-09-17 09:15   level.dat
      796  02-09-17 09:15   level.dat_old
       10  02-09-17 09:15   levelname.txt
        3  02-09-17 09:15   world_behavior_packs.json
        3  02-09-17 09:15   world_resource_packs.json
 --------                   -------
   287030                   15 files