Migrate Python package name

We now own the [prophet](https://pypi.org/project/prophet/) project on PyPI, which means we can use `prophet` as our top-level package name if we wanted.

## Proposal 1: deprecate `fbprophet` as a package name

Steps:
1. move all code to `prophet` package
2. create a second package `fbprophet` that depends on `prophet` and maintains import compatibility, issue warning at install time
3. next release: issue warning to use new package name at import time
4. 2nd release: stop updating `fbprophet` and use `prophet` exclusively.

## Proposal 2: keep `fbprophet` indefinitely and tie releases together

Everything from Proposal 1 without steps 3 and 4.

## Open Questions

- How do we make it possible for people to continue using the [old prophet package](https://prophet.readthedocs.io/en/latest/).
- Do we need to copy all old releases over to the new package name?