Fetch `tools.deps` dependencies via `clojure`, not `bazel`

[dancmeyers]

If we use standard Clojure tooling to fetch dependencies, rules_clojure becomes a lot simpler.

[dancmeyers]

There's a circular issue here where we want to generate the java_import stanzas for libgen_build etc, but we can't use gen_srcs because that needs libgen_build. We could specify them manually, but there might be minor version bumps or similar.

Maybe maven_install and pinning deps, then using Bazel to fetch them, is the better way...