BUG: resample with freq W-Mon and first data point starts on Monday doesn't respect closed

[MarcoGorelli]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd

df = pd.DataFrame({
    'ts': pd.to_datetime(['2020-01-06 00:00', '2020-01-06 03:00']),
    'values': [1, 4],
})
alias = 'W-Mon'

print(
    df.set_index('ts')
    .resample(alias, closed='right', label='right')['values'].sum()
)
print(
    df.set_index('ts')
    .resample(alias, closed='left', label='left')['values'].sum()
)

Issue Description

The above prints

ts
2020-01-06    5
Freq: W-MON, Name: values, dtype: int64
ts
2020-01-06    5
Freq: W-MON, Name: values, dtype: int64

Expected Behavior

With `closed='right' I was expecting intervals:

• interval: (2019-12-31, 2020-01-06], (2020-01-06, 2020-01-13]

With `closed='left' I was expecting intervals:

• interval: [2020-01-06, 2020-01-13)

So, my expected output is:

            values
ts                
2019-01-06       1
2020-01-13       4

Installed Versions

INSTALLED VERSIONS

commit : 965ceca
python : 3.11.3.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.0-149-generic
Version : #166-Ubuntu SMP Tue Apr 18 16:51:45 UTC 2023
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 2.0.2
numpy : 1.24.3
pytz : 2023.3
dateutil : 2.8.2
setuptools : 65.5.0
pip : 22.3.1
Cython : None
pytest : 7.3.0
hypothesis : 6.75.1
sphinx : None
blosc : None
feather : None
xlsxwriter : 3.1.2
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
brotli : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 12.0.0
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : None
snappy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None