Move more taks to core_tasks.

Review Request #3199 - Created Dec. 1, 2015 and submitted

Benjy Weinberger
jsirois, zundel

Changed a few names, and put each task in its own file,
for uniformity.

Also replaced the first=True argument to run_prep_command's
install with a comment explaining that it must be first in its, and another comment in extension_loader explaining
that core_tasks must be registered first.

The reason is that first=True was no less fragile - if anything
else registered into the 'test' goal with first=True later on,
it would preempt run_prep_command, with no comment. At least this way
the comments are clear, and we can maybe (??) move towards
deprecating first=True, or at least relying on it less.

CI passes:

Benjy Weinberger
Eric Ayers
Benjy Weinberger
Eric Ayers
Benjy Weinberger
Benjy Weinberger
Benjy Weinberger
Review request changed

Status: Closed (submitted)

Change Summary: