Use namespaces for all of the vala and genie plugins.
This was discovered when I loaded the vala and native test plugins at the same
time in gplugin query and they both tried to register types with the same
names.
Testing Done:
Ran unit tests and `gplugin-query` with all of the loaders and test plugins queryable.
Reviewed at https://reviews.imfreedom.org/r/984/
ifget_option('gtk3')
ifnotget_option('install-gplugin-gtk3-viewer')
summary('gplugin-gtk3-viewer',
'You have disabled installation of gplugin-gtk-viewer which is a '+