build: set esbuildOptions outbase to 'src' in tsup config

Set `options.outbase = 'src'` in the esbuildOptions callback to ensure
output files preserve the correct directory structure relative to the
`src` folder, preventing path flattening during the build process.

tsup.config.js

@@ -78,6 +78,7 @@ export default defineConfig([
     ],
     bundle: false, // Don't bundle these files
     esbuildOptions(options) {
+      options.outbase = 'src';
       options.loader = {
         '.js': 'jsx',
         '.jsx': 'jsx',