In DM-20994, we never quite settled on advocating specific names for the location of an instrument's FilterDefintionCollection. After some discussion, we decided it was better to make this for internal use only, and point users to the `instrument.filterDefintions` if they need to access it. This entails renaming e.g. hscFilters.py to _filters.py (or possibly _filterDefinitions.py to make it even more explicit?) and changing the filenames in creating-an-obs-package.rst to reflect this.

The obs package guide already advocates for _instrument.py as the filename that implements the Instrument class, and I think our existing gen3 obs packages follow that approach. I don't know if there are any other files that we should explicitly standardize in this manner.

I don't know if we should document in the guide above or somewhere else, how to access the gen3 Instrument in a generic manner, for code that e.g. may want access to the FilterDefinitions.