bpanel plugin build process should skip plugins that don't have expected exports

[bucko13]

If an invalid module happens to be added to the config, the build should skip it. Currently, the build will crash and none of the plugins will get built.

Steps to reproduce:

1. use bpanel-cli to install a non-plugin from npm e.g. bpanel-cli i @bpanel/bpanel-ui (or add manually to config.js)
2. wait for build to finish
3. Refresh webpage and you'll get the blue bPanel screen of death

Should be easy to invalidate in the plugin build process.