Move syncutil code into sync.py.

[vyasr]

Description

This PR merges the syncutil and sync modules.

Motivation and Context

The code in syncutil is only every used by sync, so removing this indirection makes the code more discoverable by removing an unnecessary layer of indirection.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• The package documentation and framework documentation in signac-docs are up to date with these changes.
• I have updated the changelog and added any related issue and pull request numbers for future reference.

[codecov]

Codecov Report

Merging #869 (8edb7ae) into master (35b862f) will decrease coverage by 0.01%.
The diff coverage is 80.29%.

@@            Coverage Diff             @@
##           master     #869      +/-   ##
==========================================
- Coverage   86.52%   86.51%   -0.02%     
==========================================
  Files          40       39       -1     
  Lines        4527     4522       -5     
  Branches      901      901              
==========================================
- Hits         3917     3912       -5     
  Misses        429      429              
  Partials      181      181              

Impacted Files	Coverage Δ
signac/sync.py	81.43% <80.29%> (-1.07%)	⬇️

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[bdice]

Perhaps consider reducing visibility of some public utility methods/classes.