DEPR: Index.get_loc with method
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

We only use it one place internally; this will allow us to simplify get_loc, which will marginally improve perf.