Support platform_common and ToolchainInfo

[leostera]

Hi!

I'm trying to use skydoc in some rules I'm writing for ReasonML but I'm having trouble with platform_common being undefined.

  File "external/com_github_ostera_rules_reason/reason/private/bs_module.bzl", line 127, in <module>
    providers = [platform_common.ToolchainInfo],
NameError: name 'platform_common' is not defined

From looking around I saw the fix for provider and I think I can add to the rule_extractor.py another stub for platform_common so that I can generate docs.

What do you think? :)

[laurentlb]

Yes, feel free to add more stubs.
We know it's brittle, so Skydoc is going to be rewritten in a much more robust way (cc @c-parsons).

[c-parsons]

Closing. Use Stardoc :)