Adds prompt for package name

[janash]

This PR adds a prompt for package_name, separate from repo_name. This would address issues like #82, where the error was occurring because the Python package name and the repo name were the same by default. In reality, it's totally realistic to have a repo name like project-name and a package name project_name (or something similar). The CookieCutter currently does not allow having the repo name (outer folder) and package name (inner source directory) be different.

Also addresses #154

I think I've changed most of the necessary files, but would appreciate thoughts/review 👀

[janash]

If we wanted to do this in a less aggressive but more clear way, we could also change the prompt from asking what the repo name is to asking the package name. Then it would be more clear that what is specified needs to be a valid Python package name. The repo name could be inferred based on the package name.