Recent versions of abseil strongly encourage using
Bazel modules. Therefore, switch some dependencies
over to using modules as well, but leave a few
using the old WORKSPACE syntax for now. To enable
this, we rename WORKSPACE to WORKSPACE.bzlmod
(Bazel calls this "hybrid mode").

This also involves upgrading the versions of
several of our dependencies, notably protobuf and
abseil. It would be nice to defer doing this for
now, but some initial investigation suggested that
this would be quite difficult due to the web of
dependencies.

`std::filesystem::path` is only available on macOS >= 10.15.

Not every Boost library we need is available on BCR yet (e.g., ICL),
but this seems to be good enough for now.

This reverts commit 13b244b.

There seems to be some debate about whether to add these files to git
or ignore them (e.g., [1]). For now, let's ignore them.

[1] bazel-contrib/rules_cuda#223

This appears to be necessary to workaround a bug or odd behavior in
recent versions of Bazel: in eventuals, submodules/stout was considered
to be part of the eventuals repo (despite being declared as a
local_repository) because of the renaming of WORKSPACE.bazel to
WORKSPACE.bzlmod. For the time being, re-add an empty WORKSPACE file.

This reverts commit 271adb8.

This reverts commit 62d20b2.