Support Bazel Modules and avoid using `internal` dependencies about drake

[jwnimmer-tri]

We should switch to bzlmod at some point in the future -- it's intended to be the forward-looking workspace mechanism for Bazel.

However, at the moment it is a very new feature of Bazel, and we don't yet want to force a very-new minimum Bazel version upon our users. (We currently support down to Bazel 5.1, and Bazel 7.x is barely a month old currently.)

I imagine sometime in the next six(?) months we'll switch to require Bazel >= 7.0 as our minimum (after we drop Ubuntu 20.04 support), and that would be a good time to start poking at bzlmod.

[aminya]

Note that Bazel 5 supports Bazel modules too. Bazel 7 just enforces the use of modules unless --noenable-bzlmod is passed.

https://docs.bazel.build/versions/5.0.0/bzlmod.html

[jwnimmer-tri]

My impression is that there are a non-trivial number of important fixes and features related to bzlmod that have not been backported to 5.x nor 6.x, such that trying to use bzlmod in <7.x would be a headache. If you have experience to the contrary, I'd be happy to be corrected.

[aminya]

No, I haven't used those versions as I don't see a point in sticking to old deprecated versions of technology while it is so easy to upgrade Bazel.

[jwnimmer-tri]

... it is so easy to upgrade Bazel.

We'd welcome a PR with the Bazel 7 upgrade. I did not find it to be easy to get it to pass CI, so if it's easy for you, please go ahead!