Windows path fix

[jribbink]

Closes #1183

Description

This fixes bugs related to Windows path formatting. We were using the "path" package in a few places that should have been using "path/filepath".

Some issues with this were more theoretical and didn't seem to surface yet (absolute paths like C:/foo/bar would not be recognized as absolute). But most critically the mixing of "path" and "path/filepath" meant that when doing comparisons/dictionary lookups to see if paths were equivalent the different format would result in two paths not matching (this is what broke deployment).

The PR also ensures that all paths written to flow.json are in UNIX format for cross-platform compatibility. This is the pattern adopted by NPM in the package.json file.

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

Patch coverage: 25.00% and no project coverage change.

Comparison is base (eee3195) 38.40% compared to head (819981c) 38.40%.
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1190   +/-   ##
=======================================
  Coverage   38.40%   38.40%           
=======================================
  Files          38       38           
  Lines        1940     1940           
=======================================
  Hits          745      745           
  Misses       1107     1107           
  Partials       88       88           

Flag	Coverage Δ
unittests	38.40% <25.00%> (ø)

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Files Changed	Coverage Δ
internal/super/setup.go	0.00% <0.00%> (ø)
internal/super/files.go	12.35% <25.00%> (ø)
internal/test/test.go	38.93% <33.33%> (ø)

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[sideninja]

Did a quick review. Great work. Left some comments. The most important feedback I have is to add a test that validates the persisting and loading of different path styles (unix, windows)