Fix issues loading old GGPK data in the exporter

Check version

• I'm running the latest version of Path of Building and I've verified this by checking the changelog

Check for duplicates

• I've checked for duplicate open and closed issues by using the search function of the issue tracker

What platform are you running Path of Building on?

Windows

How is Path of Building expected to behave?

When swapping the current DAT source in the exporter, we should be able to load old versions of GGPK data

How does Path of Building behave?

When we changed the path from "Data" to "Data/Balance", we aren't able to load any older GGPKs because they don't have that path available. In addition, I believe this section ends up overwriting the spec you're trying to load with the currently loaded one instead

How to reproduce the issue

Have a body of data saved from before 0.4.0, including spec.lua, data, metadata folders, etc.
Create a new Source in the exporter with that path, and switch to it to see no DAT files loaded.

PoB for PoE2 build code

Screenshots

Start in src/Export/Main.lua at the referenced lines around 380-384 and reproduce the exporter behavior with pre-0.4.0 data containing spec.lua, data, and metadata folders. Done means an older GGPK source loads its DAT files when selected and does not overwrite the selected spec with the currently loaded one.