Add a package comment for every package, and enable staticcheck's ST1000

In https://github.com/elastic/apm-server/pull/8680 we have enabled all checks in staticcheck except for [ST1000](https://staticcheck.io/docs/checks#ST1000) (Incorrect or missing package comment), as many of our packages lack package comments.

We should introduce a package comment for all packages, and enable ST1000. This should help ensure that we're a bit more intentional about the meaning of packages and how code is organised.