fix stale helper exports for build plugins

[jamilahmadzai]

Summary

This removes stale helper subpath exports from the build plugin package manifests:

• @powerlines/plugin-tsup
• @powerlines/plugin-vite
• @powerlines/plugin-tsdown
• @powerlines/plugin-esbuild
• @powerlines/plugin-rolldown

Those package.json exports point at dist/helpers/... files that are not present in the published packages or in these package source trees, so consumers can resolve a helper subpath and then fail at runtime with ERR_MODULE_NOT_FOUND.

The root exports, ./types exports, and ./package.json exports are unchanged.

Reproduction

For example, with the current published @powerlines/plugin-tsup@0.12.596 package:

mkdir -p /tmp/powerlines-plugin-tsup-pack-repro/node_modules/@powerlines
cd /tmp/powerlines-plugin-tsup-pack-repro
npm pack --silent @powerlines/plugin-tsup@0.12.596
mkdir -p node_modules/@powerlines/plugin-tsup
tar -xzf powerlines-plugin-tsup-0.12.596.tgz -C node_modules/@powerlines/plugin-tsup --strip-components=1
node --input-type=module -e "console.log(await import.meta.resolve('@powerlines/plugin-tsup/helpers'))"
node --input-type=module -e "await import('@powerlines/plugin-tsup/helpers')"

import.meta.resolve() points to dist/helpers/index.mjs, but that file is not included in the package, so importing it fails.

Validation

• Checked the changed package manifests no longer expose any ./helpers* export keys.
• git diff --check
• pnpm install --frozen-lockfile --ignore-scripts

I also attempted to run the plugin-tsup Vitest entry directly, but the test process stops during setup before executing tests because the installed @storm-software/tsup package imports @nx/js/src/utils/buildable-libs-utils without the .js extension under this Node/ESM setup. This change is limited to package export metadata.