Using explicit dependencies checks on CI

[keynmol]

1. Use https://github.com/cb372/sbt-explicit-dependencies do declare all dependencies
2. Enable undeclaredCompileDependenciesTest and unusedCompileDependenciesTest (to avoid test dependencies evicting main ones) on the CI

This can be tackled module by module, I think.

[rossabaker]

• unusedCompileDependenciesTest is already enforced.

• undeclaredCompileDependenciesTest was introduced, and backed off, in Declare more dependencies http4s#3880. A lot of the work to re-enable that is in an intermediate commit. And if we'd gone with that, we would not have had this bug.

• cats-laws shouldn't be a compile dependency of http4s-client, or most modules. That may be an orthogonal issue: I am unsure whether that triggered the test dependency to upgrade our core dependency.

[rossabaker]

The rest of the team hates this, but I still contend we're running with scissors without it.