message: docs: add some external dependency debug tips.
  These came up while chasing down
diff: @@ -70,6 +70,25 @@ The name of the dependency can be found in
  [the repository locations file.](https://github.com/envoyproxy/envoy/blob/master/bazel/repository_locations.bzl)
  The path of the local copy has to be absolute path.
  +For repositories built by `envoy_cmake_external()` in `bazel/foreign_cc/BUILD`,
  +it is necessary to populate the local copy with some additional Bazel machinery
  +to support `--override_repository`:
  +1. Place an empty `WORKSPACE` in the root.
  +2. Place a `BUILD` file with `filegroup(name = "all", srcs = glob(["**"]), visibility = ["//visibility:public"])`
  + in the root.
  +
  +# Debugging external dependencies
  +
  +For all external dependencies, overriding with a local copy as described in the
  +previous section is a useful tool.
  +
  +Below we describe specific tips for obtaining additional debug for specific
  +dependencies:
  +
  +* `libevent`: add `"EVENT__ENABLE_VERBOSE_DEBUG": "on",` to `cache_entries`
  + in the `event` target in `bazel/foreign_cc/BUILD` for verbose tracing of
  + libevent processing.
  +
  # Distdir - prefetching dependencies
  Usually Bazel downloads all dependencies during build time. But there is a
  