Unhardcode mod package loaders

[pchote]

This updates #13171 with architectural fixes and addresses the other FileSystem hacks that were blocked on having mod-defined package loaders.

This breaks the cycling dependency between assemblies that define package loaders that contain assemblies that define package loaders that... by requiring all dlls to be loaded from a normal directory. This then also fixes @abcdefg30's SharpDevelop problems.

This is already a monster of a PR, so i'm going to defer moving all the package loaders to the mod assemblies to a future PR. I've done the D2K sound parser as a testcase, but moving any more would make it difficult to keep a clean git history while addressing any review comments.

The final commit shows that it is still possible to explicitly mount subdirectories inside zip files.

I strongly suggest reviewing this commit by commit, using the ?w=1 parameter to squash whitespace changes on the main package loader commits.

[abcdefg30]

Works, thanks! I can't review the code 100%, but it looked okay as well.

[reaperrr]

I thought we dropped .oramod support, is it intentional that you keep support for that extension?

[GraionDilach]

This creeped back from the original PR I believe.

[pchote]

Good catch, fixed.

[pchote]

Fixed, rebased, and dropped the test commit.

[reaperrr]

and dropped the test commit.

That was a bit early, could you push a separate test branch? Because I can't get it to work locally, but I'm probably just making a mistake somewhere.

[reaperrr]

I can get ra|bits.zip to work but not ra|bits.zip/desert, neither on bleed nor here, though.

[pchote]

You need to do ra|bits.zip: bits and then bits|desert.

[pchote]

That was a bit early, could you push a separate test branch?

pchote@26eddcf

[reaperrr]

Test case works 👍

I'd still prefer either @RoosterDragon, @atlimit8 or someone else who is better than me at reviewing code to look over it before this is merged, so I won't merge this right away.

[reaperrr]

I gave the code another look since nobody else seems to have the time or motivation, couldn't find anything that looks wrong, and in addition to the testcase all mods still seem to work fine. I assume that if there is any bug lurking that we overlooked, we'll probably run into it before the next major release, so I'll merge this now.

[reaperrr]

https://github.com/OpenRA/OpenRA/wiki/Changelog-(bleed)/_compare/c2b9f419e339c9480db65a579d50aee6bdf2ff7c...9c8564a6cf4bc9d2ce209770a24b34ee99bd8b09