[✓][BUG] Error when loading replay for external level

[WillLem]

This bug is present in all existing versions of Lemmini (OG, SuperLemmini, SLToo, RetroLemmini).

Basically, when attempting to load a replay for an external level, we get a "level does not exist" error and the replay doesn't load. It's possible to use this workaround to get it to work, but this isn't ideal; if the external level is a test, and the plan is to later add it to a proper pack, the replay file would have to be edited again.

So, the plan is to fix this for RetroLemmini. We want to first check the levels folder for a match, and if none is found, check the external levels folder for a level matching the name specified in the replay file. If one is found, attempt to load the replay as normal.

[WillLem]

Fixed in commit 982196f. We now fallback to the external levels folder if no match is found in the main level pack index.