Store ModeState.fields into a dict.

[Simon Charette]

ModeState initially stored its fields into a List[Tuple[str, models.Field]] because ​it wanted to preserve ordering.

However the auto-detector doesn't consider field re-ordering as a state change and Django doesn't support table column reordering in the first place. The only reason I'm aware of for keeping field ordering is to generate model forms out of them which is unlikely to happen during migrations and if it was the case the only the order in which field are ordered and validated would change if Meta.fields = '__all__ is used ​which is discouraged.

Given storing fields this way results in awkward and inefficient lookup by name for no apparent benefits and that dict now preserves insertion ordering I suggest we switch ModelState.fields to Dict[str, models.Field]. I suggest we do the same for ModelState.indexes and .constraints since they suggest from the same awkwardness which was likely cargo culted from ModelState.fields design decision.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 696024fb:

Refs #31499 -- Ignored field ordering to determine ModelState equality.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 06889d62:

Fixed #31499 -- Stored ModelState.fields into a dict.

This allows the removal of its O(n) .get_field_by_name method and many
other awkward access patterns.

While fields were initially stored in a list to preserve the initial
model definiton field ordering the auto-detector doesn't take field
ordering into account and no operations exists to reorder fields of a
model.

This makes the preservation of the field ordering completely superflous
because field reorganization after the creation of the model state
wouldn't be taken into account.