Adopt afero, create tests for globby.

[nathanhammond]

This PR does three things:

• Adopt afero inside of the globby module. This is a nice isolated test of afero, which we can expand into the rest of the codebase if we remain happy with it.
• Writes a bunch of tests of the globby implementation in order to attempt to specify the behavior prior to rewriting the implementation of GlobFiles.
• Removes the implicit dependency on os.Getwd in one call to GlobFiles.

I've added a review for questions.

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[nathanhammond]

The Windows CI failure is legitimately failing because of path separator differences. I expected them to be normalized and they ... weren't.

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[gsoltis]

I'm super excited for this. I think this will be a major upgrade for the codebase.

Addressed.

[gsoltis]

One extra idiomatic Go thing, which I apologize for not catching earlier: you almost always want to use foo := thing instead of var foo = thing. The style guide covers it here

And actually, having now read that section, I suspect I am not 100% following it, specifically the piece about using var for empty slices. I'm not 100% sold on their argument, but if we're going to try to follow the guide, I guess we ought to move in that direction going forward.

Happy to have a good shared rant about Go sometime in an unrecorded medium.

Addressed.

[nathanhammond]

I went back and addressed all the style guide issues I introduced here.