Add locate command

[kiritofeng]

• Add locate command: output is

Skipped self-tests

Running local judge...

/tmp/problems/a
/tmp/problems/b
c is not a valid problem, skipping

for file structure

/tmp/problems » tree
.
├── a
│   └── init.yml
└── b
    └── init.yml

3 directories, 2 files

• Add typing annotations to dmoj/judgeenv.py
• Make get_problem_root return None if the problem does not exist
• Fix typo exposed in dmoj/commands/resubmit.py exposed by typing dmoj/judgeenv.py
• Fix typing annotation in dmoj/packet.py: os.path.getmtime() returns a float not int.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0f534a0) 81.91% compared to head (433e9f5) 81.30%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1126      +/-   ##
==========================================
- Coverage   81.91%   81.30%   -0.61%     
==========================================
  Files         141      141              
  Lines        5026     5028       +2     
==========================================
- Hits         4117     4088      -29     
- Misses        909      940      +31     

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[quantum5]

Seems mostly legit.