Restrict user dir loading to the correct subfolders

[SJuliez]

This was requested by Hammer a while ago: loading content from the userdir now requires it to be placed in the same folder structure that is used internally even for those files where it wouldnt technically be necessary like .mtf or .ttf. Units must now be somewhere in userdir/data/mekfiles etc.

[pavelbraginskiy]

Wait this sucks. Why would we want that.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.32%. Comparing base (95af77b) to head (9c72635).
Report is 15 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6757      +/-   ##
============================================
- Coverage     29.33%   29.32%   -0.01%     
+ Complexity    15600    15594       -6     
============================================
  Files          2864     2864              
  Lines        279533   279586      +53     
  Branches      49326    49337      +11     
============================================
- Hits          82004    82000       -4     
- Misses       191975   192033      +58     
+ Partials       5554     5553       -1     

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[Sleet01]

This is going to cause massive user grief, and that's just describing how I personally will feel.
If we don't provide users with a way to set this up, we're going to get tons of bug reports when their existing user folders stop working for them.

What is the desired end result from this change?

[HammerGS]

Simple, we have players that have dropped stuff assuming it works where ever it works then it doesn't. They come to Discord and we have to spend a lot of time troubleshooting. Where if we had a uniform consistent stucture the question becomes. Did you put it in this folder.

It also maintains a consistency with the existing folders which lets us easier explain how to set up the folders.

[HammerGS]

Going to convert to a draft, till we discuss.

[gsparks3]

At a minimum, if a matching folder structure is to be required, I would want some way of automatically generating the correct folder structure within a selected user folder (just the empty folders, in which various files are to be placed, but without having to manually create the entire folder tree and maybe make a bunch of typos).

[JonhyBurner]

At a minimum, if a matching folder structure is to be required, I would want some way of automatically generating the correct folder structure within a selected user folder (just the empty folders, in which various files are to be placed, but without having to manually create the entire folder tree and maybe make a bunch of typos).

I like this, because it also immediately tells the user exactly what folders are supported.