(packing) Ensure the checksum for .mdl, .phy and .vtx are matching for each prop

[maxdup]

(low priority, nice to have) The goal is to ensure there is no conflicts in the case where people have multiple versions of the same prop under the same name (same internal path). Checking the checksum would ensures that the packing won't mix and match files of different versions (altho this is unlikely to happen in any sane folder structure). Mixing and matching prop files from different props under the same name will create errors when loading assets ingame.

https://developer.valvesoftware.com/wiki/MDL https://developer.valvesoftware.com/wiki/VTX https://developer.valvesoftware.com/wiki/PHY