How do a BuildToolPlugin source to source transformation?

I have a Swift PM target, here called XYZ, which contains a single Swift file. Through a BuildToolPlugin I would like to modify the original Swift source file and include the generated Swift source output in the build: original.swift -> (BuildTooldPlugin) -> modified.swift.

The issue I'm running into right now is that the definitions of the original and the generated Swift file get included such that the build fails on: Invalid redeclaration of ...

How can I exclude the original unmodified source file from the build to prevent the duplicate def issue?

Target making use of the plugin:

    name: "XYZ",
    dependencies: [],
    path: ".",
    sources: ["XYZ"],
    plugins: [.plugin(name: "XYZPlugin")]

The plugin:

struct XYZPlugin: BuildToolPlugin {
    func createBuildCommands(
        context: PackagePlugin.PluginContext,
        target: PackagePlugin.Target
    ) async throws -> [PackagePlugin.Command] {
        guard let target = target as? SourceModuleTarget else { exit(1) }
        let sources: [Path] = target.sourceFiles.filter { $0.type == .source }.map(\.path)

        return { input in
            let output = context.pluginWorkDirectory.appending(["\(input.stem)Modified.swift"])

            return .buildCommand(
                displayName: "XYZ: \(input)",
                executable: Path("\("),
                arguments: [input, output],
                inputFiles: [input],
                outputFiles: [output]

Same as here: BuildToolPlugin: How to explicitly define template files in package manifest?. The .swift input file can be defined within createBuildCommands.