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/