Allow external codemods to be registered via setuptools entry points

I'm looking at shipping some codemods in https://github.com/HypothesisWorks/hypothesis/issues/2705, to automatically upgrade uses of deprecated or just plain inefficient APIs, making it easier to migrate to newer versions of Hypothesis.

One thing I've noticed is that the *end user* has to explicitly declare where the codemods can be found - in my case, adding a list entry

```yaml
# List of modules that contain codemods inside of them.
modules:
- 'libcst.codemod.commands'
- 'hypothesis.extra.codemods'
```

I think it would be a better end-user experience if I could register the codemods from Hypothesis though, and I'd therefore like to propose [using setuptools entry points](https://amir.rachum.com/blog/2017/07/28/python-entry-points/) as another source of modules to search for codemods. Then

```python
# In Hypothesis' setup.py
entry_points={
"pytest11": ["hypothesispytest = hypothesis.extra.pytestplugin"],
"console_scripts": ["hypothesis = hypothesis.extra.cli:main"],
"libcst_codemod_modules": ["_= hypothesis.extra.codemods"],
},
```

and a small amount of code in LibCST, and we're done!

(for reference, almost the code in [Hypothesis' plugin loader](https://github.com/HypothesisWorks/hypothesis/blob/master/hypothesis-python/src/hypothesis/entry_points.py) is just to get `importlib.metadata`, `importlib_metadata`, or `pkg_resources` to give us the list in a generally compatible way... but you can reuse that logic, at least!)