plugin-so: use list of components instead of glib hash table

Use a list of components to track those that have references to their so
handles. This allows removing a dependency on the glib hash table, which
is not very cooperative when initialized from a library constructor in a
static build (mainly on Windows).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>