Simplify latesttag branch with early return, drop lifetime workaround

[Copilot]

The latesttag special case in canonicalize_location used a pre-declared resolved_tag variable (with a "must outlive the match below" comment) to work around borrow lifetimes — unnecessary since there's no match after it, just a direct Ok(...) return.

Changes

• Early return in latesttag branch: replaces mutation of selector/location_type with a direct return Ok(("gittag".into(), url.into(), tag))
• Drop mut: location_type and selector no longer need to be mutable

// Before
let resolved_tag; // must outlive the match below
if location_type == "latesttag" {
    resolved_tag = Self::resolve_latest_tag(url, selector)?;
    selector = resolved_tag.as_str();
    location_type = "gittag";
}
Ok((location_type.into(), url.into(), selector.into()))

// After
if location_type == "latesttag" {
    let tag = Self::resolve_latest_tag(url, selector)?;
    return Ok(("gittag".into(), url.into(), tag));
}
Ok((location_type.into(), url.into(), selector.into()))

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