feat(filesystem): Implement access to shadowed files inside archives

[xezon]

This change implements access to shadowed files inside archives.

The Archive File System already kept track of all archives regardless of the shadowing, but made shadowed files inaccessible. This is now changed, which allows for some special uses cases we will run into when dealing with shadowed files of any kind. Shadowing is especially prevalent in Zero Hour, because that shadows a large amount of Generals files.

The review is quite a bit intricate given the complexity of the matter, the convuluted EA code and use of the uncommon multimap.

What is a shadowed file?

Consider the following files:

C:\GeneralsZH\art\textures\house.dds
C:\GeneralsZH\TexturesZH.big         -> contains art\textures\house.dds
C:\Generals\Textures.big             -> contains art\textures\house.dds

C:\GeneralsZH\art\textures\house.dds shadows art\textures\house.dds in C:\GeneralsZH\TexturesZH.big

art\textures\house.dds in C:\GeneralsZH\TexturesZH.big shadows art\textures\house.dds in C:\Generals\Textures.big

What is changed?

File instances can now be accessed by their instance index:

Instance 0 : C:\GeneralsZH\art\textures\house.dds
Instance 1 : C:\GeneralsZH\TexturesZH.big         -> contains art\textures\house.dds
Instance 2 : C:\Generals\Textures.big             -> contains art\textures\house.dds

[xezon]

This change is not adding instance access to this function. I think it opens a potential rabbit hole and increase the complexity of this change.

[xezon]

Something worthwhile to investigate. I did not intent to fix it in this change because the potential side effects are unknown.

[Skyaero42]

Ok