Refactor: Rename spellchecker.py to _spellchecker.py

Per review comment.

Author: nthykier

codespell_lib/_codespell.py

@@ -39,17 +39,17 @@
     Tuple,
 )
 
+from ._spellchecker import (
+    DetectedMisspelling,
+    LineTokenizer,
+    Spellchecker,
+)
 from ._text_util import fix_case
 
 # autogenerated by setuptools_scm
 from ._version import (  # type: ignore[import-not-found]
     __version__ as VERSION,  # noqa: N812
 )
-from .spellchecker import (
-    DetectedMisspelling,
-    LineTokenizer,
-    Spellchecker,
-)
 
 word_regex_def = r"[\w\-'’]+"  # noqa: RUF001
 # While we want to treat characters like ( or " as okay for a starting break,