Fix broken bzl_library build graph

[cvarier]

What type of PR is this?

• Bug fix

What does this PR do? Why is it needed?
Currently, //go/private:context is missing a dep on //go/private/rules:transition, which is necessary in order to load @io_bazel_stardoc//stardoc:stardoc.bzl. This will be used in a subsequent PR for adding targets to generate Stardoc output.

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[cvarier]

@googlebot I signed it!

[robfig]

Thanks for the fix! Can you also add a command that would have identified this problem to our CI script, to prevent regression?

[alexeagle]

@robfig something would have to evaluate the starlark load statements to detect a missing edge in the graph, and @cvarier and I are working on #2437 which would add such a regression test.

The only alternative I can think of is to introduce a usage of gazelle in this repo to generate the bzl_library targets, and then on CI assert that gazelle finds no changes to make. But that seems hard per https://github.com/bazelbuild/rules_go/blob/master/BUILD.bazel#L40-L42

[robfig]

OK, thanks!