REF: move offset_to_period_map from liboffsets (pandas-dev#34447)

Author: jbrockmendel

pandas/_libs/tslibs/offsets.pyx

@@ -30,7 +30,7 @@ from pandas._libs.tslibs.util cimport is_integer_object, is_datetime64_object
 from pandas._libs.tslibs.base cimport ABCTimestamp
 
 from pandas._libs.tslibs.ccalendar import (
-    MONTHS, DAYS, MONTH_ALIASES, MONTH_TO_CAL_NUM, weekday_to_int, int_to_weekday,
+    MONTH_ALIASES, MONTH_TO_CAL_NUM, weekday_to_int, int_to_weekday,
 )
 from pandas._libs.tslibs.ccalendar cimport get_days_in_month, dayofweek
 from pandas._libs.tslibs.conversion cimport (
@@ -45,53 +45,6 @@ from pandas._libs.tslibs.tzconversion cimport tz_convert_single
 
 from .timedeltas cimport delta_to_nanoseconds
 
-# ---------------------------------------------------------------------
-# Constants
-
-_offset_to_period_map = {
-    'WEEKDAY': 'D',
-    'EOM': 'M',
-    'BM': 'M',
-    'BQS': 'Q',
-    'QS': 'Q',
-    'BQ': 'Q',
-    'BA': 'A',
-    'AS': 'A',
-    'BAS': 'A',
-    'MS': 'M',
-    'D': 'D',
-    'C': 'C',
-    'B': 'B',
-    'T': 'T',
-    'S': 'S',
-    'L': 'L',
-    'U': 'U',
-    'N': 'N',
-    'H': 'H',
-    'Q': 'Q',
-    'A': 'A',
-    'W': 'W',
-    'M': 'M',
-    'Y': 'A',
-    'BY': 'A',
-    'YS': 'A',
-    'BYS': 'A'}
-
-need_suffix = ['QS', 'BQ', 'BQS', 'YS', 'AS', 'BY', 'BA', 'BYS', 'BAS']
-
-for __prefix in need_suffix:
-    for _m in MONTHS:
-        key = f'{__prefix}-{_m}'
-        _offset_to_period_map[key] = _offset_to_period_map[__prefix]
-
-for __prefix in ['A', 'Q']:
-    for _m in MONTHS:
-        _alias = f'{__prefix}-{_m}'
-        _offset_to_period_map[_alias] = _alias
-
-for _d in DAYS:
-    _offset_to_period_map[f'W-{_d}'] = f'W-{_d}'
-
 
 # ---------------------------------------------------------------------
 # Misc Helpers

pandas/tseries/frequencies.py

@@ -5,13 +5,18 @@
 
 from pandas._libs.algos import unique_deltas
 from pandas._libs.tslibs import Timestamp
-from pandas._libs.tslibs.ccalendar import MONTH_ALIASES, MONTH_NUMBERS, int_to_weekday
+from pandas._libs.tslibs.ccalendar import (
+    DAYS,
+    MONTH_ALIASES,
+    MONTH_NUMBERS,
+    MONTHS,
+    int_to_weekday,
+)
 from pandas._libs.tslibs.fields import build_field_sarray
 from pandas._libs.tslibs.offsets import (  # noqa:F401
     DateOffset,
     Day,
     _get_offset,
-    _offset_to_period_map,
     to_offset,
 )
 from pandas._libs.tslibs.parsing import get_rule_month
@@ -39,6 +44,51 @@
 # ---------------------------------------------------------------------
 # Offset names ("time rules") and related functions
 
+_offset_to_period_map = {
+    "WEEKDAY": "D",
+    "EOM": "M",
+    "BM": "M",
+    "BQS": "Q",
+    "QS": "Q",
+    "BQ": "Q",
+    "BA": "A",
+    "AS": "A",
+    "BAS": "A",
+    "MS": "M",
+    "D": "D",
+    "C": "C",
+    "B": "B",
+    "T": "T",
+    "S": "S",
+    "L": "L",
+    "U": "U",
+    "N": "N",
+    "H": "H",
+    "Q": "Q",
+    "A": "A",
+    "W": "W",
+    "M": "M",
+    "Y": "A",
+    "BY": "A",
+    "YS": "A",
+    "BYS": "A",
+}
+
+_need_suffix = ["QS", "BQ", "BQS", "YS", "AS", "BY", "BA", "BYS", "BAS"]
+
+for _prefix in _need_suffix:
+    for _m in MONTHS:
+        key = f"{_prefix}-{_m}"
+        _offset_to_period_map[key] = _offset_to_period_map[_prefix]
+
+for _prefix in ["A", "Q"]:
+    for _m in MONTHS:
+        _alias = f"{_prefix}-{_m}"
+        _offset_to_period_map[_alias] = _alias
+
+for _d in DAYS:
+    _offset_to_period_map[f"W-{_d}"] = f"W-{_d}"
+
 
 def get_period_alias(offset_str: str) -> Optional[str]:
     """