Fix findManifestTargetCall to only search top-level targets array

[krishnapermi]

When swift package add-target-dependency is used on a target whose name also appears as a dependency of another target, the refactoring incorrectly modifies the dependency reference instead of the actual target definition — producing a malformed Package.swift.

Root cause: findManifestTargetCall used FirstNodeFinder which walks the entire syntax tree recursively. Because a target's name string (e.g. .target(name: "TargetOne")) can appear both as a top-level target definition and nested inside another target's dependencies: array, the recursive search finds the nested occurrence first and returns the wrong node.

Fix: Replace the recursive findFirst search with a linear scan over the top-level elements of the targets: array, so only target definitions are considered — not dependency references nested inside them.

A regression test is added that covers the exact scenario from the linked bug report.

Fixes swiftlang/swift-package-manager#10122