fix: DefinitionIds should not be stored in id_to_type

[jfecher]

Description

Problem*

Resolves the underlying issue with #4207 where definition/expr types are being overwritten by other exprs/definitions because DefinitionIds are converted into Indexes which may clash with those from expressions.

Summary*

Separates out id_to_type into another map for only definitions: definition_to_type to resolve the clash. I've also removed the impl From<DefinitionId> for Index to prevent this issue in the future.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[michaeljklein]

LGTM