Change: move iterators to dedicated target files

[mattmundell]

What

Move target iterators out of manage_sql.c.

Why

Reducing size of manage_sql.c. Better file organisation.

References

Follows /pull/2768.

Testing

Ran GET__TARGETS with branch and main, then compared output.

o m m '<get_targets details="1" tasks="1"/>' > /tmp/t-main
o m m '<get_targets details="1" tasks="1"/>' > /tmp/t-pr
o m m '<get_targets trash="1"/>' > /tmp/t-trash-main
o m m '<get_targets trash="1"/>' > /tmp/t-trash-pr
diff -u /tmp/t-pr /tmp/t-main
diff -u /tmp/t-trash-pr /tmp/t-trash-main