Pretest dependencies

[Riolku]

This PR changes the previous "batch dependencies" in a few ways:

1. Dependencies can now be specified on any top-level unit, case or batch.
2. Dependencies are specified with YAML alias notation, not numbers.

It also implements pretests, including a PRETEST-END IPC packet.

[Xyene]

We'll need an accompanying site-side PR to not crash the bridge when we send it PRETEST-{BEGIN,END} packets.

https://github.com/DMOJ/online-judge/blob/f3cd24e3ecad89c2492917df9229009c2634baed/judge/bridge/judge_handler.py#L40-L54

[Riolku]

Also have dependencies always be a set instead of a list that gets coerced to a set over and over.

[codecov-commenter]

Codecov Report

Base: 80.75% // Head: 53.87% // Decreases project coverage by -26.87% :warning:

Coverage data is based on head (5001269) compared to base (ae6b204). Patch coverage: 79.01% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1075       +/-   ##
===========================================
- Coverage   80.75%   53.87%   -26.88%     
===========================================
  Files         134      134               
  Lines        4770     4850       +80     
===========================================
- Hits         3852     2613     -1239     
- Misses        918     2237     +1319     

Impacted Files	Coverage Δ
dmoj/judge.py	32.56% <72.80%> (-20.77%)	:arrow_down:
dmoj/problem.py	91.72% <92.85%> (+1.10%)	:arrow_up:
dmoj/graders/base.py	89.79% <94.73%> (+11.74%)	:arrow_up:
dmoj/config.py	82.71% <100.00%> (+0.21%)	:arrow_up:
dmoj/executors/C.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/D.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/GO.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/ADA.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/AWK.py	0.00% <0.00%> (-100.00%)	:arrow_down:
dmoj/executors/C11.py	0.00% <0.00%> (-100.00%)	:arrow_down:
... and 83 more

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