Fix rpak aliasing not working when trying to alias towards a vanilla rpak

[ASpoonPlaysGames]

Fixing stuff described by @Alystrasz on discord. I don't really like this behaviour as it behaves a bit differently for vanilla rpak loads and modded rpak loads, but aliasing is hacky already so meh.

[GeckoEidechse]

cc @Alystrasz for testing

[Alystrasz]

Explanation from Spoon on Discord:

it only worked before because of two bugs in the old system

which meant that when you aliased an rpak, the game would look in the vanilla folder for it first instead of the modded one, but then when it tried to load the vanilla one, it loaded the modded one as well

new system only looks in the modded folder for the alias, so for s2s where it is trying to alias a non-existent rpak to a vanilla rpak that behaviour is broken

[Alystrasz]

To test this change, I installed s2space, a mod aliasing vanilla rpaks; then launched a game in private match using map mp_s2s.

Without PR

With PR

[uniboi]

Code looks good. Maybe a comment that mentions that vanilla aliases behave a bit different?

[GeckoEidechse]

cc @ASpoonPlaysGames to decide whether to merge with or without comment

[ASpoonPlaysGames]

Dont think it's necessary, it acts fairly intuitively

[GeckoEidechse]

ah fuck, forgot to prefix commit with core:. Sorry @F1F7Y :c