Support starlark lock file

[sluongng]

It's pretty common in the Bazel ecosystem to use starlark to manage configuration.

A starlark struct could be composed programatically and then loaded elsewhere in the build graph (i.e., augmenting a test target).

[markelliot]

We generate a list of the available tools in @<hub_repo>//:tools.bzl, does that work for you?