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>
3 jobs for task-names in 8 minutes and 20 seconds
Status Job ID Name Coverage
  Build
failed #810609
debian-stable-x86_64

00:01:02

passed #810554
fedora-x86_64

00:07:18

failed #810555
debian-stable-x86_64

00:00:40

 
Name Stage Failure
failed
debian-stable-x86_64 Build
Uploading artifacts...
_build/config.h: found 1 matching files

_build/meson-logs: found 2 matching files

WARNING: _build/debian-stable-x86_64-report.xml: no matching files
Uploading artifacts to coordinator... ok
id=810609 responseStatus=201 Created token=f1c86Snu
Uploading artifacts...
WARNING: _build/debian-stable-x86_64-report.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1