Format imports with GCI (#407)

`golangci-lint` includes `gci` by default, so we can disable
`goimports`. The auto-fix function still works, so `make lintfix` is
enough to reformat any files with incorrectly grouped imports.

.golangci.yml

@@ -36,6 +36,7 @@ linters:
     - funlen            # rely on code review to limit function length
     - gocognit          # dubious "cognitive overhead" quantification
     - gofumpt           # prefer standard gofmt
+    - goimports         # rely on gci instead
     - golint            # deprecated by Go team
     - gomnd             # some unnamed constants are okay
     - ifshort           # deprecated by author