Rename CLI and Python packages

[sgreenbury]

Part of #77.

[sgreenbury]

The current packages are now renamed as:

• popgetter -> popgetter-core
• popgetter_cli -> popgetter-cli
• popgetter_py -> popgetter-py

One result of the rename is that none of the crates share the name of the project (i.e. popgetter). There are a few options if we'd like to have a popgetter crate:

1. Revert popgetter-core to popgetter
2. Change popgetter-cli to popgetter
3. Create a workspace level crate re-exporting the member crates (possibly with features). This would lead to 4 crates: popgetter-core, popgetter-cli, popgetter-py, popgetter (workspace level)
4. Create another member crate called popgetter providing with popgetter-core as a dep and providing a crate with the sole purpose of being a Rust library for popgetter (e.g. like the bindings packages: Python, npm etc)

I think (for now) it might make sense to retain flexibility and keep the renames as is but could consider implementing/changing to any the above options in the future.

[sgreenbury]

Merging this now following discussion with @andrewphilipsmith with a popgetter crate as a placeholder or providing docs pointing to the 3 popgetter crates planned in #77.