CI currently enforcing formatting rules by using the fmt linter of golang-ci-lint which is invoked from the bin/lint script. However it doesn't seem possible to use golang-ci-lint as a formatter, only as a linter which checks formatting. This means any formatter used by your IDE or invoked manually may or may not use the same formatting rules as golang-ci-lint depending on which formatter you use and which specific revision of that formatter you use.
In this change we stop using golang-ci-lint for format checking. We introduce `tools.go` and add goimports to the `go.mod` and `go.sum` files. This allows everyone to easily get the same revision of goimports by running `go install -mod=readonly golang.org/x/tools/cmd/goimports` from inside of the project. We add a step in the CI workflow that uses goimports via the `bin/fmt` script to check formatting.
Some shell gymnastics were required in the `bin/fmt` script to work around some limitations of `goimports`:
* goimports does not have a built-in mechanism for excluding directories, and we need to exclude the vendor director as well as the generated Go sources
* goimports returns a 0 exit code, even when formatting errors are detected
Signed-off-by: Alex Leong <alex@buoyant.io>
Alex Leong