Groupby apply result shape depends on internal apply path
#### Problem description

The shape returned by groupby.apply currently depends on the path the internal apply takes (fast apply vs slow apply) which is opaque to the user. The following examples show the behaviour for two simple examples where the fast path returns the group key as index while the slow path returns the original index.

#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

df = pd.DataFrame({'A': [0, 0, 1], 'b': range(3)})

def slow(group):
       # slow apply because of check `result is input`, c.f. https://github.com/pandas-dev/pandas/blob/44782c0809e296a8e57b7f77d963e999c7e0f4a7/pandas/_libs/reduction.pyx#L494
       return group

def fast(group):
       return group.copy()

```
```
   ...: df.groupby("A").apply(slow)
   ...:
Out[3]:
   A  B
0  0  0
1  0  1
2  1  2

In [4]: df.groupby("B").apply(fast)
Out[4]:
     A  B
B
0 0  0  0
1 1  0  1
```

#### Expected Output

A transparent and consistent output data type. In particular, the behaviour should not be coupled to private, performance related internals.

#### Related issues

There are a lot of related issues which may or may not have the same root cause. Here an excerpt

#12824
#14538
#12977
#28662

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.1.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 1.0.0
numpy            : 1.16.5
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.1.0.post20200119
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
