Add golangci-lint, cleanup code

[LandonTClipp]

Description

• Add golangci-lint with an initial set of linters
• Modify mockery config to generate all mocks (this is to allow checking the generation of all mocks in CI/CD)
• Rename/reshuffle location of some fixtures for more clarity
• Fix some bugs caught by the linters

Fixes #590

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Version of Golang used when building/testing:

• 1.11
• 1.12
• 1.13
• 1.14
• 1.15
• 1.16
• 1.17
• 1.18
• 1.19
• 1.20

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Patch coverage: 11.76471% and project coverage change: -0.54324 ⚠️

Comparison is base (6f08a5d) 75.62432% compared to head (1c3e1ab) 75.08108%.

❗ Current head 1c3e1ab differs from pull request most recent head 89f6865. Consider uploading reports for the commit 89f6865 to get more accurate results

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master        #591         +/-   ##
===================================================
- Coverage   75.62432%   75.08108%   -0.54324%     
===================================================
  Files              7           7                 
  Lines           1842        1850          +8     
===================================================
- Hits            1393        1389          -4     
- Misses           363         371          +8     
- Partials          86          90          +4     

Impacted Files	Coverage Δ
cmd/mockery.go	63.44411% <0.00000%> (-0.99359%)	⬇️
pkg/config/config.go	70.08929% <0.00000%> (-1.08188%)	⬇️
pkg/outputter.go	41.20603% <0.00000%> (+0.20602%)	⬆️
pkg/generator.go	92.85714% <25.00000%> (-0.36996%)	⬇️
pkg/walker.go	62.83186% <25.00000%> (-2.03300%)	⬇️

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