Initial open-sourcing of add_dep tool

[cushon]

See bazelbuild/bazel#4990

This is a lightly modified fork of the internal tool, with updates to e.g. use bazel instead of blaze, and use the standard flags package, etc. I confirmed that it works on examples like bazelbuild/bazel#4990.

One open question here is test coverage. The internal implementation has a suite of e2e integration tests that can be run manually. They set up a tiny bazel workspace, do a build that produces an add_dep error, and confirm that running the provided command fixes the build. I could look at externalizing those. It would be nicer to have something that could run on CI.

[sluongng]

you probably want to move each of these files to a separate directory to follow the convention of 1 dir == 1 package. Or to rename all the packages to add_dep.
This way the repo can still be built using go install.

[sluongng]

Suggested change

	load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library", "go_test")
	load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")