Support statically linked module plugins

Up til now, QQmlImports would always assume module plugins are
dynamically linked, and as such, go looking for them on the file
system. This would fail for plugins linked in statically.

This patch will hook into QQmlImports at the place where it
iterates through all plugins in a qmldir and try to load them
dynamically. If some plugins cannot be resolved, we now do an
extra interation over static plugins to check if any
of them has the correct uri specified in their metadata.

Hooking into the loading process this late, will ensure that as
much code as possible is shared between dynamic and static plugin loading
so that setting up base urls, namespaces and guards will remain the same.

Note: this patch is one out of several patches that is needed to build
QML2 apps statically, and depends on plugins reporting their URI
through the plugin meta data system. This will be injected
automatically by qmake+moc.

Task-number: QTBUG-28357
Change-Id: If9a204e942ca7003448e188a1a47eec69b34c37b
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>