RFC: spread field maps validation logic into smaller functions

[neutrinoceros]

The underlying motivation is to get rid of the one warning I get from running pyright on main as

uv run --with pyright pyright src/gpgi

/Users/clm/dev/gpgi/src/gpgi/_data_types.py
  /Users/clm/dev/gpgi/src/gpgi/_data_types.py:130:52 - error: "_reference_field_name" is possibly unbound (reportPossiblyUnboundVariable)
1 error, 0 warnings, 0 informations 

Although the code is actually correct, it's understandable that a static analysis cannot prove it, and I want static analysis to prove correctness so let's rewrite it.
It's also arguably much nicer to remove 2 levels of indentation