Use "append" method to construct the various game paths
instead of wasteful string concatenation. Additionally, use a
temporary to extract and reuse a result of a few common
subexpressions to further reduce the overhead.
Reserve space for the list of games in findWorldSubgame. The
performance gain is pretty much negligible but this change
also gets rid of a performance warning by CLANG TIDY.
This patch will make distinguishable mods in modpacks possible in the future
`nil` checks are required to provide backwards-compatibility for fresh configured worlds
