release: get semrel to tag submodules

[fastcat]

The manually-triggered Release GHA flow will make a new tag/release for the root module, but we also want it to (intelligently) make tags/releases for the sub-modules. This requires a bunch of customization to get semrel to look only for changes in the subdir, and apply the prefix to the new tag/release.

[fastcat]

a094b1a gets a rudimentary version of this going. A full solution involves checking the commit messages only for the affected folders. This is, in principle, relatively easy to do for submodules, but more annoying to do for the root due to the need to exclude the submodules.