WIP: 🌱 Enable gci linter

[stmcginnis]

What this PR does / why we need it:

This adds the gci linter to make sure imports are properly formatted and
grouped. This will remove the need for code reviewers to nitpick PRs by
making it an objective result reported from a test.

Which issue(s) this PR fixes:
Related #4622

Hitting some failures when running make test that I need to investigate yet.
Getting errors like:

2021/05/18 17:17:16 http: TLS handshake error from 127.0.0.1:51246: EOF

Followed by failures from 500 results and other mismatches.

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[sbueringer]

@stmcginnis I think we only wanted to add: exportloopref, ifshort, nilerr (#4624 (comment))

Although I personally really like that one and the grouping with local imports :)

[stmcginnis]

Thanks Stefan. I'll close this and maybe we can decide if we want to reopen it later. I do think it adds some nice checks that could help avoid nitpicking in commits.

At least it uncovered that we have a bunch of importing errors. There are a surprising number of cases where we import the same package multiple times in a file under different aliases. In some cases this looks like maybe just mistakes as different people have worked on the same file, but in others they were very legitimate errors where we were originally using two different packages but accidentally updated to use the same one. (#4632)

[sbueringer]

@stmcginnis Nice! I add a proposal about importas linter to the issue, it addresses a related issue