Better support for stardoc usage in another directory from the bzl files

It seems like it would make sense to put your `stardoc()` rules in a `docs` directory where you check in the generated files. But to do that you also have to export the `*.bzl` files if it has dependencies because the `bzl_library` will have more that one file in it. Ideally the rule would support a `bzl_library` as input where there is only one direct src entry, or it should support a raw string that isn't a label so the `deps` of the `bzl_library` could tell it which file to act on.

Start by locating the stardoc() rule and bzl_library handling referenced in the issue, then trace how direct src entries and deps are processed. Done means stardoc can be used from a separate docs directory without exporting dependency .bzl files, with coverage for the supported input form.

Written by the indexing model from the issue text.