Automatically update bazel_dep(...) dependencies

[npaun]

I pasted in an old get_bcr_version() function I did last summer, and then asked Claude to implement the rest of the logic to implement bazel_dep directives.

Note that this still doesn't handle cases where we have bazel_dep + archive_override, or other kinds of overrides. Initially, this was going to be trivial, but we had to create a module extension to inject our proxy for the internal repo, so we cannot literally have update-deps.py generate that syntax anymore. I'll need to do some tinkering when I get back from vacation to force Bazel to accept this.