here's the performance regressions, they affect any callers of `distributions()` and are even worse on callers of the new apis.

a call to distributions() is about 3x slower than in 3.9

here is the setup I am using:

virtualenv venv39 -ppython3.9
venv39/bin/pip install flake8 pytest twine pre-commit
virtualenv venv310 -ppython3.10
venv310/bin/pip install flake8 pytest twine pre-commit

to test just the `distributions()` call I'm using the following:

```console
$ venv39/bin/python -m timeit -n 20 -r 20 -s 'from importlib.metadata import entry_points' 'entry_points()'
20 loops, best of 20: 12.5 msec per loop
$ venv310/bin/python -m timeit -n 20 -r 20 -s 'from importlib.metadata import entry_points' 'entry_points()'
20 loops, best of 20: 36.7 msec per loop
```

this is a less-extreme example, many applications have more dependencies installed -- but even in this case this is adding ~24ms startup to any application using `entry_points()` -- and it gets worse

the return value of `entry_points()` alone isn't all that useful, next an application needs to retrieve its entry points.  let's start for the somewhat normal case of retrieving a single category of entry points:

```console
$ venv39/bin/python -m timeit -n 20 -r 20 -s 'from importlib.metadata import entry_points' 'entry_points()["flake8.extension"]'
20 loops, best of 20: 12.7 msec per loop
$ venv310/bin/python -m timeit -n 20 -r 20 -s 'from importlib.metadata import entry_points' 'entry_points(name="flake8.extension")'
20 loops, best of 20: 37.1 msec per loop
$ venv310/bin/python -m timeit -n 20 -r 20 -s 'from importlib.metadata import entry_points' 'entry_points().select(group="flake8.extension")'
20 loops, best of 20: 37.1 msec per loop
```

again, 3x slower and very real time to the end user (~24-25ms)

now let's show an example usage that something like flake8 uses where multiple groups are requested (this is common for apps and plugin systems which provide multiple distinct functionalities)

```console
$ venv39/bin/python -m timeit -n 20 -r 20 -s 'from importlib.metadata import entry_points' 'eps = entry_points(); eps["flake8.extension"]; eps["flake8.report"]'
 
20 loops, best of 20: 12.6 msec per loop
$ venv310/bin/python -m timeit -n 20 -r 20 -s 'from importlib.metadata import entry_points' 'eps = entry_points(); eps.select(group="flake8.extension"); eps.select(group="flake8.report")'
20 loops, best of 20: 38.2 msec per loop
```

also slower, but an additional ms per call to `.select(...)`


and it only gets worse with more and more packages installed

here's the versions I'm using to ensure they are up to date:

```console
$ venv39/bin/python --version --version
Python 3.9.5 (default, May 19 2021, 11:32:47) 
[GCC 9.3.0]
$ venv310/bin/python --version --version
Python 3.10.0b2 (default, Jun  2 2021, 00:22:18) [GCC 9.3.0]
```