Move external-appstream plugin and appstream utils into core

Following on from !779 (merged), this is another load of refactoring which moves the external-appstream plugin, and the gs-appstream.c utility code, into the core of gnome-software. Again, the idea here is to reduce interactions between plugins so that threading can be refactored in future. In this case, the commits here are steps towards merging the appstream plugin itself into the core, making it explicit that appstream data is a core part of gnome-software. When gnome-software was originally written, appstream was a lot younger and less widely adopted — we can now take advantage of the fact it’s more universal. That will remove the dependency between a lot of plugins and the appstream plugin, by eventually (in a subsequent MR) moving the appstream plugin into the plugins which use it.

Signed-off-by: Philip Withnall pwithnall@endlessos.org