Plugins are not run when cache is valid

[thieman]

AutoDLL seems to bail out from the compile completely when it has a warm cache to work from. This extends to skipping the execution of any defined plugins in the AutoDLL config.

We have some plugins that we need to execute even if the cache is valid, in particular we are using an assets manifest that will fail our build in production if not all files are included.

I'm not knowledgeable enough to suggest exactly how this might look, but some way to designate a subset of plugins to run even in the cache case would be useful. For now, we have written a small plugin of our own to rebuild our assets manifest from cache when necessary.