bug(developer): removal of .keyboard_info from source means no way to split e.g. `und-fonipa` vs `und-latn`

sil_ipa currently has `und-fonipa` as its langtag in .keyboard_info, which is the most appropriate tag. However, for compatibility reasons (e.g. #10754), the .kps lists `und-Latn` (which has its own problem #10727 but that's beside the point).

The problem is that we have eliminated .keyboard_info from the source of keyboards with 17.0. This means that there is no way to maintain this distinction. Potential fixes:

1. Include a fixup in the keyboard compiler to convert tags that we know will cause trouble to only include subtags in the holy lang-script-region triplet (minimizing as possible of course).
2. Just use `und-latn` in the keyboard search and shrug
3. Include the correct tag in .kps and .kmp, and include a fixup in all Keyman platforms for known problem cases (similar fixup to pt 1 above but at keyboard install time).
4. Use both 1 and 3 -- for v17 target keyboards, use pt 3, but all other keyboards use pt 1.