And other simplifications

Thanks to jb.

We need to select a format that can be output by the mixer, preferably
the exact format output by the mixer. Keeping the input audio format
breaks for non-linear input formats when S/PDIF output is disabled:
the mixer outputs FL32 that cannot be converted to MPGA, AC-3, etc.

There are two advantages:
 - scanning uncached plugins is much faster,
 - plugins with broken dependencies are visible in the modules list.

Plugins are still resolved "now" if they are probed by module_need().
This is a safety feature (the run-time linker would exit silently if it
hit an unresolved symbol). As the previous commit unloads all uncached
plugins, we can reload the plugins with different flags as needed.

Plugins with broken/missing underlying libraries will trigger an error
only when used, rather than when scanned. vlc-cache-gen is then a bit
more robust against messed up packages installation (it will not skip
plugins anymore). Also, dialog_Fatal() could be used to report run-time
errors. This was not possible previously: the error would occur before
the UI was started.

Note that this is not implemented on Windows, as there is no support
(that I know) for lazy resolution of DLLs.

This saves about 20 Mb of address space, 10 Mb of memory on my system,
when the plugins cache is disabled. It is even more dramatic for
non-VLC apps (as then Qt4 is not loaded).

Each cache file is now unloaded as soon as the cache directory scan is
complete. The data is not needed anymore past that point.

As the module descriptor is referenced into the main module list
(without copying).