Python: introduce toPythonApplication function

This commit introduces the `toPythonApplication` function. Certain
Python packages are considered both a library and an application, that
is, they expose importable modules, but typically executables that are
part of the package are used instead.

In this case, the package needs to be added to `python-packages.nix` in
order for it to be available as a library. An alias with this function
can then be added in `all-packages.nix`, e.g.:

```
ansible = with pythonPackages; toPythonApplication ansible;
```

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