gs-plugin-loader: Set GTask names

This makes the tasks a little easier to debug, especially in sysprof
output. Calling `g_task_set_source_tag()` sets the task name to the
stringified version of the source tag.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
2 jobs for task-names in 4 minutes and 34 seconds
Status Job ID Name Coverage
  Build
passed #819134
debian-stable-x86_64

00:01:32

passed #819133
fedora-x86_64

00:04:34