Make real directory paths on the VFS predictable.

Partial revert/fix of 2f19cf86d3 and 2567fee329.

Before this diff, it was possible for a mod containing a cache/ or a
config/ folder to get written to incorrectly.
The issue is VFS can map multiple directories to one 'virtual' path, for
reading mods. However, writing data is problematic: which path to
choose?
The only viable solution is to use a path relative to the highest
priority directory encountered in the VFS path, or write paths could be
'hijacked' by lower-priority mods.

This fixes these issues by:
- Adding a new lookup mode ('Real-path') that explicitly picks the real
path relative to the highest-priority subdirectory in the VFS Path.
- Preventing overwriting a real directory with a lower priority one in
general.
- Revert c0c8132dd4's GetRealPath change, re-introducing the function as
GetOriginalPath.

This also cleans up some duplication that led to empty mod folders in
the user mod path, and cleans up loading the 'user' mod.

Note that the new 'realpath' lookup can still be somewhat complex with
many mount points at various hierarchy levels, but it is at least
predictable/deterministic without having to be careful about populating
order.

Fixes #2553

Differential Revision: https://code.wildfiregames.com/D3217
This was SVN commit r25104.

source/ps/CacheLoader.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2018 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -140,7 +140,7 @@ VfsPath CCacheLoader::LooseCachePath(const VfsPath& sourcePath, const MD5& initi
 
 	// Get the mod path
 	OsPath path;
-	m_VFS->GetRealPath(sourcePath, path);
+	m_VFS->GetOriginalPath(sourcePath, path);
 
 	return VfsPath("cache") /
 		path_name_only(path.BeforeCommon(sourcePath).Parent().string().c_str()) /