Gazelle flat mode

[jayconrod]

Gazelle should be able to generate a single flat BUILD.bazel file for all targets in a repository. This file may be customized (if needed) and used with new_git_repository.

This issue tracks work that must be done in Gazelle before go_repository can be deprecated (#460). Flat mode will be behind an experimental flag at first. Later, when Gazelle takes responsibility for adding and updating external dependencies, this mode will be turned on by default for third party repos.