Check if a `package_dist_name` is taken in PyPI when filling out the cookiecutter

[bielsnohr]

This came as a point of feedback from the UKAEA Coding Discussion Group session on 2021-09-24. It would be nice if there was an automatic check whether the package_dist_name clashes with an already existing package name in PyPI rather than the user having to go and manually check. It would like be quite difficult to do this dynamically (i.e. while the user is typing), but possible to validate the input once it has been entered and perhaps issue a warning.