[bazel] OpenTitan as a Bazel module dependency

[jwnrt]

Description

It would be useful to allow other Bazel modules to depend on OpenTitan in order to access our toolchain, software, tools, etc. for things like downstream demos, operating systems, etc.

There are some issues preventing this:

1. OpenTitan is not in the central registry.

   • Projects can work around this using git_override, however this can only be used in root modules.

2. For some reason Bazel only allows include directives in the root MODULE.bazel and not those of dependencies.

   • This is surprising and seems like an arbitrary restriction.
   • We may have to remove our use of include which may complicate our repo.

3. @engdoreis has found that the .llvm extension from https://github.com/bazel-contrib/toolchains_llvm that we use to get a toolchain for bindgen can only be used from the root module.

   • No idea why. Maybe we can replace this with the lowRISC RISC-V toolchain? We'd need to get an x86_64 libc++ from somewhere still.

4. @engdoreis also found that we cannot use @// in the MODULE.bazel, it should be @@// or just //. That's an easy fix.

[jwnrt]

CC @pamaury and @nbdd0121, do you have any thoughts on the include problem? Would it be too messy to inline rust.MODULE.bazel and python.MODULE.bazel into the main MODULE.bazel?

[pamaury]

I also do not understand this limitation of include. I think putting everything into one file would be very messy. An idea would be to generate MODULE.bazel from a template where the includes are replaced by textual inclusion, check-in that file. This would make MODULE.bazel very messy but this would not be the main file that people edit?

[jwnrt]

That could work, but I think the extra step required to generate MODULE.bazel would cancel out the simplification of being able to split out sub-files.