filter-apply: Mark modules as being autoloaded

(Based on Colin's review) We mark modules as being autoloaded so that
they can handle this as a special case if needed (which is required by
module-echo-cancel for now). This inverts how things were done and makes
using these modules manually less error-prone.

src/modules/module-virtual-source.c

@@ -69,6 +69,9 @@ PA_MODULE_USAGE(
 struct userdata {
     pa_module *module;
 
+    /* FIXME: Uncomment this and take "autoloaded" as a modarg if this is a filter */
+    /* pa_bool_t autoloaded; */
+
     pa_source *source;
     pa_source_output *source_output;