`HasSource::source` should be infallible

[Veykril]

None of the implementations ever return None, so it should not return an option either. https://github.com/veykril/rust-analyzer/blob/9476fdaaa96323800b43c528e59617b14908d6a2/crates/hir/src/has_source.rs#L23

[Veykril]

A lot of TryToNav impls can probably move over to the infallible ToNav version as well as a follow up

[tetsuharuohzeki]

I would like to take this.

However, HasSource::source has been changed to return Option once in #6913.
Was the motivation in #6913 outdated?

[Veykril]

Oh, I forgot about that. Hmm having it return an Option does make sense then given we might want to support rlibs someday. We should at least have a comment on the trait then describing this design.

[tetsuharuohzeki]

@Veykril I open #15140 to add why we use Option for HasSource::source