Add visibility extension to support recursive default_visibility (#783)

[dnathe4th]

Under a new /bazel/ subdirectory, this language-agnostic extension allows a directive to control the default_visibility templated out to all BUILD.bazel files in or under the directory. This feature is intended to allow large codebases to define hierarchical visibility defaults.

What type of PR is this?

Uncomment one line below and remove others.

Feature

What package or component does this PR mostly affect?

language/

Which issues(s) does this PR fix?

Fixes #783