Allow tree-shaking of frontend plugins

Adding rollup's preserveModules to enable tree-shaking in webpack.

Signed-off-by: Tomasz Szuba <tszuba@box.com>

.changeset/green-adults-push.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Add preserveModules to rollup, which allows better async loading and tree-shaking in webpack

packages/cli/src/lib/builder/config.ts

@@ -100,6 +100,7 @@ export async function makeRollupConfigs(
         chunkFileNames: `esm/[name]-[hash].esm.js`,
         format: 'module',
         sourcemap: true,
+        preserveModules: true,
       });
       // Assume we're building for the browser if ESM output is included
       mainFields.unshift('browser');