.rename() does not check conflict with multiindex level names

[hazbottles]

MCVE Code Sample

>>> import xarray as xr
>>> ds = xr.Dataset({"x": 42}, {"level0": [0, 1], "level1": [3, 4]}).set_index(mindex=["level0", "level1"])
>>> ds
<xarray.Dataset>
Dimensions:  (mindex: 2)
Coordinates:
  * mindex   (mindex) MultiIndex
  - level0   (mindex) int64 0 1
  - level1   (mindex) int64 3 4
Data variables:
    x        int64 42
>>> ds.rename({"x": "level0"})
<xarray.Dataset>
Dimensions:  (mindex: 2)
Coordinates:
  * mindex   (mindex) MultiIndex
  - level0   (mindex) int64 0 1
  - level1   (mindex) int64 3 4
Data variables:
    level0   int64 42

Expected Output

raise a ValueError("conflicting Multiindex names ...")

Problem Description

Using .rename() you can get into a situation where you have a data variable or coordinate with the same name as a multi-index level name.

Output of xr.show_versions()

INSTALLED VERSIONS

commit: b3d3b44
python: 3.8.0 | packaged by conda-forge | (default, Nov 22 2019, 19:11:38)
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 4.4.0-18362-Microsoft
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: None
libnetcdf: None

xarray: 0.14.1+36.gb3d3b44
pandas: 0.25.3
numpy: 1.18.0
scipy: None
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.9.1
distributed: 2.9.1
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
setuptools: 42.0.2.post20191201
pip: 19.3.1
conda: None
pytest: 5.3.2
IPython: None
sphinx: None