qtdeclarative5-qtmultimedia-plugin needs to depend on libqt5multimedia5-plugins

For the Ubuntu Touch Music app to work, we need to add an extra dependency to libqt5multimedia5-plugins in addition to the existing qtdeclarative5-qtmultimedia-plugin one.

It'd be good if that dependency was already provided in qtdeclarative5-qtmultimedia-plugin

Here's an extract of a conversation about it on #ubuntu-app-devel

<dpm> seb128, it seems the music app already depends on qtdeclarative5-qtmultimedia-plugin - shall we just add a dependency on libqt5multimedia5-plugins then?
<seb128> dpm, I'm not familiar enough with that stack to say, to me it looks like you are just using qtdeclarative5-qtmultimedia-plugin ... which should bring on the required plugins
 dpm, I hit the same issue with system settings yesterday when adding the sound preview feature
 dpm, imho it's qtdeclarative5-qtmultimedia-plugin that should get the depends
 dpm, but check with somebody who knows qtmultimedia better
<dpm> seb128, so you think I should file a bug to request qtdeclarative5-qtmultimedia-plugin to depend on libqt5multimedia5-plugins ?
<seb128> yes
 seems like to me that qtdeclarative5-qtmultimedia-plugin doesn't work out of the box currently
 it doesn't make sense to start adding that depends in every qml apps using qtmultimedia