fix(sourcemaps): Fix double association bug

[szokeasaurusrex]

Fixes a bug where the same sourcemap can be associated with mulitple source files.

Change description

We solve this problem by keeping track of the sourcemaps we have already associated with a source file. If we guess a sourcemap which has already been associated, we do not associate the given source file with any sourcemap.

Also, we iterate first through all the sources which explicitly reference the sourcemap – i.e., for which, we don't have to guess the sourcemap. These are associated first. As they are explicitly linked to a sourcemap, we intentionally do not prevent duplicates here.

On the second iteration, we guess the sourcemap reference for all remaining sources. We enforce the no duplicates rule on the second iteration. Sources cannot be associated with a sourcemap that was explicitly referenced by another source on the first iteration. If multiple sources guess the same sourcemap, and that sourcemap is not explicitly associated with any source, then none of the sources will be associated with that sourcemap (as it is unclear how that sourcemap should be associated).

In all cases, we warn the users that we failed to associate the sources properly.

Issues

Fixes #2445
Fixes CLI-5

[linear]

CLI-5 Ensure `collect_sourcemap_references` does not associate the same sourcemap with multiple source files

[lcian]

The high level idea and implementation look good to me after carefully reading through the code.

I would suggest simplifying the code to prioritize ease of understanding. It's easy to lose track when there's a lot of nesting like this.

Maybe some of the processing logic can be extracted into functions.
If this is not possible/beneficial, we could at least add comments to the for_each/fold/filter steps where the body is multiple lines, to make it easier to reason about specific steps in isolation.

[szokeasaurusrex]

Converting to draft while I work on the simplification, per @lcian's request

[szokeasaurusrex]

@lcian is this good now?

[lcian]

Looks good and much easier to understand, thanks!