move "vet" checks from golangci-lint to nogo

[jrockway]

Nogo is rules_go's way of linting go code. It runs during compilation, and only during compilation. This means that lint results are cached along with compiled libraries, so if a library isn't being compiled, you don't have to spend time linting it.

Moving govet is the easiest linter. I plan to gradually chip away at everything that golangci-lint does, but for now, we're just getting the simplest one.

(Additionally, this makes bazel builds behave more like go builds. Go runs vet whenever you request a test, and won't run the test if vet fails. This doesn't happen with Bazel, but golangci-lint would have gotten it. Now it works again, and also affects non-test targets, which I think is helpful.)

We also gain the ability to write our own lint rules: https://github.com/bazelbuild/rules_go/blob/master/go/nogo.rst#writing-and-registering-analyzers

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fe35f29) 58.72% compared to head (b39f610) 58.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9671      +/-   ##
==========================================
+ Coverage   58.72%   58.95%   +0.23%     
==========================================
  Files         582      582              
  Lines       71045    71045              
==========================================
+ Hits        41720    41884     +164     
+ Misses      28708    28534     -174     
- Partials      617      627      +10     

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