feat(gazelle): resolve class-level deps via external CrossResolvers

[shs96c]

External Gazelle extensions (e.g. a Wire or third-party protobuf generator) can already provide package-level Java resolutions via resolve.CrossResolver. They cannot provide class-level resolutions, so a split package, where some classes of a Java package come from an external extension's target and hand-written sources in the same package are built by a java_library/kt_jvm_library this extension generates, can't be resolved correctly.

The hand-written source registers the package in Gazelle's global index, so package resolution returns a single match. Gazelle only consults CrossResolve on a zero-result index miss, so the external co-provider is never seen and the split goes undetected.

Precedence is preserved: # gazelle:resolve directive > in-repo class > CrossResolver.

External-plugin contract

An external extension contributes class-level resolutions by implementing resolve.CrossResolver and answering dotted fully-qualified class names for lang == "java":

func (e *myExtension) CrossResolve(c *config.Config, ix *resolve.RuleIndex, imp resolve.ImportSpec, lang string) []resolve.FindResult {
    if imp.Lang != "java" {
        return nil
    }
    if target, ok := e.classToTarget[imp.Imp]; ok { // imp.Imp e.g. "com.example.foo.MyMessage"
        return []resolve.FindResult{{Label: target}}
    }
    return nil
}

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