refactor(python): compute relative-import dot prefix explicitly

[dbrattli]

Follow-up cleanup to #4803 (which fixed #4797).

#4803 fixed an off-by-one in the number of leading dots for relative imports of nested files with a minimal one-line change. This PR refactors MakeImportPath's relativePath so the fix is structural rather than incidental.

What changed

The old code mapped each .. segment to its own dot and relied on String.concat "." inserting a separator per join to add the rest — so the leading-dot count emerged from the join rather than being computed. That coupling is exactly what produced the original off-by-one (....sibling.api instead of ...sibling.api for imports two levels up).

The refactor computes the prefix explicitly — one dot for the package, plus one per parent level (..) — and joins only the module-name segments:

let markers, names = parts |> Array.partition (fun p -> p = "." || p = "..")
let prefix =
    if isLibrary && markers.Length > 0 then
        let parents = markers |> Array.filter ((=) "..") |> Array.length
        String.replicate (parents + 1) "."
    else ""

It also adds a docstring with worked examples.

Behavior

This is a behavioral no-op relative to main (i.e. relative to #4803): identical output wherever the old code was correct, and the fix is correct at any nesting depth by construction. Verified against representative inputs:

parts	output
["."; "module"]	.module
[".."; "sibling"; "api"]	..sibling.api
[".."; ".."; "sibling"; "api"]	...sibling.api
[".."; ".."; ".."; "s"; "api"]	....s.api
[".."; "My.Dir"; "api"]	..My_Dir.api (dir dots → _, #3079)

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