public int getFirstThunk() {
        return firstThunk;
    }
This field is an offset (an RVA) to an 
IMAGE_THUNK_DATA union. In almost every case, 
the union is interpreted as a pointer to an 
IMAGE_IMPORT_BY_NAME structure. If the field 
isn't one of these pointers, then it's supposedly 
treated as an export ordinal value for the DLL 
that's being imported. It's not clear from the 
documentation if you really can import a function 
by ordinal rather than by name. 
@return an offset (an RVA) to an IMAGE_THUNK_DATA union
Ghidra/Features/Base/src/main/java/ghidra/app/util/bin/format/pe/ImportDescriptor.java