skip_existing_chunks field in exploration_map item modifier is named incorrectly

The skip_existing_chunks field in exploration_map is named incorrectly because it does allow picking structures from existing chunks. This should be renamed to indicate that it skips already-located structures.

Expected behavior
"skip_existing_chunks": true causes the exploration map to point to a new chunk.

Actual behavior
"skip_existing_chunks": true causes the exploration map to point to a structure not previously found by exploration maps. They might or might not be in existing chunks. Internally, serialized structure start's references field is checked; this integer will increment each time the structure is "referenced" through exploration maps, and skip_existing_chunks makes it so that it only locates structure starts with 0 references.