hardcoded references to `hack` folder

[ab9rf]

There are several places in DFHack that assume that the hack folder is in the current working directory, an assumption that we can no longer assume to be true. These hardcoded references need to be rewritten to use Core::getInstance().getHackPath().

There is one library reference:

• library/DFSteam.cpp

There are three plugins:

• plugins/dig.cpp
• plugins/orders.cpp
• plugins/pathable.cpp

I didn't find any in Lua scripts - it appears that all Lua scripts are already using getHackPath

[ab9rf]

On further investigation i did find several scripts with hardcoded hack references. These have been addressed in #5753 and in DFHack/scripts#1569. Further investigation may still be needed.