This introduces a dedicated compound type for module containers
(i.e. plug-in shared libraries or statically linked module
descriptors), or plug-in. A plug-in can contain any number of
modules. The type describes plug-ins regardless of whence they came
from, libvlccore ("main"), static linking, dynamic linking or the
plug-ins cache.

Previously, plug-ins from the cache were described by module_cache_t,
only during the loading phase, while module_t was (ab)used to describe
loaded or cached plug-ins. Ultimately, that allows moving properties of
plug-ins out of the module structure where they do not really belong,
since there can be multiple modules per plug-ins.