[Flight] Basic scan of the file system to find Client modules (#20383)

* Basic scan of the file system to find Client modules

This does a rudimentary merge of the plugins. It still uses the global
scan and writes to file system.

Now the plugin accepts a search path or a list of referenced client files.
In prod, the best practice is to provide a list of files that are actually
referenced rather than including everything possibly reachable. Probably
in dev too since it's faster.

This is using the same convention as the upstream ContextModule - which
powers the require.context helpers.

* Add neo-async to dependencies