chore: check for references to private functions during path resolution

[TomAFrench]

Description

Problem*

Resolves

Summary*

This PR pulls out the actual changes to path resolution from #4491, omitting the changes which add visibility modifiers to modules.

We now determine whether a function is private (and so a warning or error should be emitted) during def collection. This is necessary as to properly determine whether a function is public or private we need information on whether all the modules in the function's path relative to where it's being used allow its contents to be used from this module - information which doesn't exist in the typechecking pass.

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.