[swift-inspect] use CMake FetchContent to fetch ArgumentParser when not provided

[andrurogerz]

Purpose

Simplify CMake build of swift-inspect by using FetchContent to fetch and build swift-argument-parser when not provided manually via ArgumentParser_DIR=....

Overview

• Replace CMake find_package with FetchContent with FIND_PACKAGE_ARGS, which will first try find_package as before but will fetch from remote if not provided
• Specify swift-argument-parser v1.5.0 since it builds properly for Android
• Update Packages.swift to also specify swift-argument-parser v1.5.0 for consistency
• Update README to reflect the change

Validation

• CMake build for Android arm64-v8a on Windows 11 with and without ArgumentParser_DIR
• CMake build for Windows x64 on Windows 11 with and without ArgumentParser_DIR
• CMake build for Linux x86_64 on Fedora 41 with and without ArgumentParser_DIR

[compnerd]

LGTM; we should check with @mikeash about bumping the s-a-p dependency.

[compnerd]

@swift-ci please test