refactor!: reorganize source code

[chingor13]

Cleans up tests to use sinon instead of proxyquire

BREAKING CHANGE: This does not actually break any interfaces that are intended to be public (exported in index.ts), but would be breaking if you are importing from deeply nested paths which is not intended.

[codecov]

Codecov Report

Merging #208 (5ea549b) into master (3d4aa4b) will decrease coverage by 0.39%.
The diff coverage is 95.84%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
- Coverage   88.04%   87.65%   -0.40%     
==========================================
  Files          24       17       -7     
  Lines        2301     2211      -90     
  Branches      168      168              
==========================================
- Hits         2026     1938      -88     
+ Misses        275      273       -2     

Impacted Files	Coverage Δ
src/bin/handle-git-dir-change.ts	79.28% <0.00%> (ø)
src/github/branch.ts	100.00% <ø> (ø)
src/github/commit-and-push.ts	97.84% <ø> (ø)
src/github/fork.ts	100.00% <ø> (ø)
src/github/labels.ts	100.00% <ø> (ø)
src/utils/diff-utils.ts	97.36% <ø> (ø)
src/bin/workflow.ts	83.67% <60.00%> (-0.71%)	⬇️
src/github/review-pull-request.ts	95.10% <95.10%> (ø)
src/github/open-pull-request.ts	100.00% <100.00%> (ø)
src/index.ts	98.70% <100.00%> (-0.03%)	⬇️
... and 4 more

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