Refactor _extract_global_metadata() (#452)

This PR does 3 things:

1. Makes `_extract_global_metadata()` private
2. Removes the part in the definition where it adds a `CURIE_MAP` since
this is subsequently popped off
3. Updates the return type to be more accurate

src/sssom/util.py

@@ -27,7 +27,6 @@
  COLUMN_INVERT_DICTIONARY,
  COMMENT,
  CONFIDENCE,
- CURIE_MAP,
  MAPPING_JUSTIFICATION,
  MAPPING_SET_ID,
  MAPPING_SET_SOURCE,
@@ -68,7 +67,7 @@
  get_converter,
 )
 from .sssom_document import MappingSetDocument
-from .typehints import MetadataType, PrefixMap, get_default_metadata
+from .typehints import MetadataType, get_default_metadata
 
 logging = _logging.getLogger(__name__)
 
@@ -149,8 +148,7 @@ def from_mapping_set_document(cls, doc: MappingSetDocument) -> "MappingSetDataFr
  return cls(df=pd.DataFrame(), converter=doc.converter)
 
  df = pd.DataFrame(get_dict_from_mapping(mapping) for mapping in doc.mapping_set.mappings)
- meta = extract_global_metadata(doc)
- meta.pop(CURIE_MAP, None)
+ meta = _extract_global_metadata(doc)
 
  # remove columns where all values are blank.
  df.replace("", np.nan, inplace=True)
@@ -1010,14 +1008,13 @@ def get_file_extension(file: Union[str, Path, TextIO]) -> str:
  return "tsv"
 
 
-def extract_global_metadata(msdoc: MappingSetDocument) -> Dict[str, PrefixMap]:
+def _extract_global_metadata(msdoc: MappingSetDocument) -> MetadataType:
  """Extract metadata.
 
  :param msdoc: MappingSetDocument object
  :return: Dictionary containing metadata
  """
- # TODO mark as private
- meta = {CURIE_MAP: msdoc.prefix_map}
+ meta = {}
  ms_meta = msdoc.mapping_set
  for key in [
  slot