Use C++17 std::optional<T> in return type of unexceptionally-fallible functions about libloot HOT 2 CLOSED

I'm going to undo the change to GameInterface::GetPlugin(), as std::shared_ptr<> already handles the null state effectively, and usage in the LOOT application has shown that wrapping it in an optional doesn't provide any benefits, it just creates extra work.

I'm also flip-flopping on DatabaseInterface::GetPluginMetadata() and DatabaseInterface::GetPluginUserMetadata(), as optionals overlap with PluginMetadata::HasNameOnly(), which when true means there's a metadata object that contains no metadata. Unfortunately this is a valid state as the metadata files may have an entry with only a name field, unless that becomes invalid syntax, so HasNameOnly() needs to stay and you end up having to check if the optional is nullopt and is HasNameOnly() is true.

from libloot.

Done as of 1fe8f90.

from libloot.