Makes the config module internal

[vyasr]

Description

This PR moves signac.config to signac._config, reflecting that all its members are private. To simplify its usage, all the functions that it contains have had their leading underscores removed since they are now redundant.

Motivation and Context

Resolves #874

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 #877 (6bbfd86) into master (16a33c7) will increase coverage by 0.10%.
The diff coverage is 86.95%.

@@            Coverage Diff             @@
##           master     #877      +/-   ##
==========================================
+ Coverage   86.51%   86.61%   +0.10%     
==========================================
  Files          39       41       +2     
  Lines        4522     4580      +58     
  Branches      901      901              
==========================================
+ Hits         3912     3967      +55     
- Misses        429      432       +3     
  Partials      181      181              

Impacted Files	Coverage Δ
signac/_config.py	90.47% <ø> (ø)
signac/__main__.py	77.76% <80.00%> (+0.03%)	⬆️
signac/migration/v1_to_v2.py	88.37% <100.00%> (ø)
signac/project.py	89.96% <100.00%> (ø)
signac/errors.py	93.18% <0.00%> (ø)
signac/__init__.py	100.00% <0.00%> (ø)

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[b-butler]

I just have one comment about our import in signac/project.py.

[b-butler]

👍