How can I use a Swift package build plugin to process a custom resource?

I am using Vapor and its FileMiddleware to host CSS files. It automatically hosts all files in my packages Public directory. I want to pre-process my CSS files using Tailwind CSS, so I am trying to create a build tool plugin to do so.

I've implemented my plugin like this:

import PackagePlugin

struct TailwindCSSBuild: BuildToolPlugin {    
    func createBuildCommands(context: PluginContext, target: Target) throws -> [Command] {
        guard let target = target as? SourceModuleTarget else { return [] }
        let inputFiles = target.sourceFiles.filter({ $0.path.extension == "css" })
        return try { inputFile in
            let inputPath = inputFile.path
            let outputName = inputPath.stem + ".css"
            let outputPath = context.pluginWorkDirectory.appending(outputName)
            return .buildCommand(
                displayName: "Generating \(outputName) from \(inputPath.lastComponent)",
                executable: try context.tool(named: "tailwindcss").path,
                arguments: [
                    "-i", "\(inputPath)",
                    "-o", "\(outputPath)",
                    "-c", "\("tailwind.config.js"))"
                inputFiles: [inputPath],
                outputFiles: [outputPath]

Whenever I call swift run with my app package that uses the plugin, I get

error: unexpected input file: <path to my app package>/.build/plugins/outputs/app/App/TailwindCSSBuild/index.css

Is there a way to do custom processing of a resource like this without throwing this error?