refactor: split pipeline.py into per-stage package (#17)

The monolithic onecite/pipeline.py (~3000 lines) is replaced with a proper package:

  onecite/pipeline/

    __init__.py       - re-exports + `requests` at package level

    _utils.py         - _safe_year helper

    parser.py         - ParserModule

    identifier.py     - IdentifierModule (largest stage, kept as one file)

    enricher.py       - EnricherModule

    formatter.py      - FormatterModule

Backward-compat is preserved:

  * `from onecite.pipeline import IdentifierModule` still works

  * `patch('onecite.pipeline.requests.get', ...)` still works because

    __init__.py keeps `import requests` at package level and Python's

    module cache means all child modules share the same `requests` object.

Tests:

  * 3 tests in test_pipeline_unit.py that used `patch.object(pipeline_mod, 'scholarly', ...)` now patch the concrete submodule (identifier / enricher) where `scholarly` is imported.

  * test_integration.py gained a missing `import pytest` so pytest.skip() works when the mocked first pass returns no results.

Author: Ang

onecite/pipeline/__init__.py

@@ -0,0 +1,34 @@
+# !/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+"""OneCite's 4-stage processing pipeline.
+
+Historically this lived in a single ``pipeline.py`` of ~3000 lines.  It was
+split per pyOpenSci review issue #17 into one module per stage.  All public
+symbols are re-exported here so callers and tests that do
+
+    from onecite.pipeline import IdentifierModule
+    import onecite.pipeline as pm  # and then: patch("onecite.pipeline.requests.get", ...)
+
+keep working unchanged.
+"""
+
+# Keep ``requests`` at package level so that tests which do
+# ``patch("onecite.pipeline.requests.get", ...)`` resolve the attribute
+# correctly.  Because Python caches modules, this is the same ``requests``
+# module object that all sub-modules import — so the patch reaches them too.
+import requests  # noqa: F401
+
+from ._utils import _safe_year
+from .parser import ParserModule
+from .identifier import IdentifierModule
+from .enricher import EnricherModule
+from .formatter import FormatterModule
+
+__all__ = [
+    "ParserModule",
+    "IdentifierModule",
+    "EnricherModule",
+    "FormatterModule",
+    "_safe_year",
+]

onecite/pipeline/_utils.py

@@ -0,0 +1,16 @@
+# !/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+"""Small shared helpers used by more than one pipeline stage."""
+
+
+def _safe_year(date_obj):
+    """Safely extract year from a CrossRef date object like {'date-parts': [[2015, 3, 1]]}."""
+    if not date_obj:
+        return None
+    parts = date_obj.get('date-parts', [])
+    if parts and isinstance(parts, list) and len(parts) > 0:
+        inner = parts[0]
+        if isinstance(inner, list) and len(inner) > 0:
+            return inner[0]
+    return None