Instead of hand-managing and running linters, use golangci-lint.

Along with the golangci-lint defaults,
enable a couple other linters we generally agree with.
See also uber-go/zap#1323 for a similar change.

As a result of this, we can:

- Drop the dependabot for tools
- Run the lint job in parallel with build/test
- Simplify the Makefile

Updates all Go dependencies in root and tools/.

Supersedes #96, #94, #91

Bumps [github.com/mgechev/revive](https://github.com/mgechev/revive) from 1.3.1 to 1.3.2.
- [Release notes](https://github.com/mgechev/revive/releases)
- [Changelog](https://github.com/mgechev/revive/blob/master/.goreleaser.yml)
- [Commits](https://github.com/mgechev/revive/compare/v1.3.1...v1.3.2)

---
updated-dependencies:
- dependency-name: github.com/mgechev/revive
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Fixes up the `make lint` target to also check
that all files are properly gofmt-ed,
replaces the deprecated `golint` check with `revive`,
and sets up dependabot to also update tools and actions regularly.

Upgrades to latest release of Go for CI.
This requires switching staticcheck to the latest release as well.

This simplifies the Makefile significantly,
borrowing patterns we've used in other projects.
Namely:

- Set GOBIN to a bin subdirectory
  so that we can `go install` dependencies into it.
- Use a shared TEST_FLAGS for `make test` and `make cover`.
  Without this, we're not running with data race detection in CI.
- Build lint step out of separate golint and staticcheck steps.
  In the future, a gofmt step may also be added.
- Move tools dependencies into an unpublished subpackage.

Note: I didn't mess with the 'clean' and 'run' targets
at the bottom of the file even though they're not necessary
to avoid a merge conflict with #60.