Simplify Pkg_rules.source_files a bit

[ElectreAAS]

Just a trivial simplification in Pkg_rules.source_files, nothing changing dune behaviour.

Instead of mapping the relative part later, do it once in the partition.
This isn't particularly useful on its own, but in case of changes inside the partition function, it makes debugging a lot easier
Extracted from #13792

[Alizter]

I think this if needs to be pulled out above the let releative. We always consider skipping, but we don't always need to construct relatative.

[ElectreAAS]

You mean something like this:

List.rev... ->
match kind with
| S_DIR -> if skip_dir name then Skip else Right (Path.Local.relative path name)
| _ -> if skip_file name then Skip else Left (Path.Local.relative path name))

It'd go against the idea of not repeating the relative call, for what I perceive to be minimal benefit

Did you miss that skip_dir and skip_file are different functions?

[Alizter]

That's true, my bad.

[Alizter]

I've asked copilot to review in case it notices anything important.

[Copilot]

Pull request overview

This PR refactors Pkg_rules.Pkg.source_files to compute Path.Local.relative once during directory partitioning, keeping behavior the same while simplifying downstream mapping and making future debugging inside the partition function easier.

Changes:

• Compute relative paths once in the rev_filter_partition_map callback and carry Path.Local.t values forward.
• Simplify accumulation by removing of_list_map and eliminating an extra relative mapping step for subdirectories.

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