Support being built without ostree support

We're close to stabilizing the composefs backend officially, and for people using that from the start, they may not want the ostree backend at all. We should support being built without it.

Start by locating the build configuration and dependency declarations that include ostree, then trace how the composefs backend is selected. Verify the project can build with ostree support disabled while retaining a usable composefs backend, and run the relevant build and test checks.

Written by the indexing model from the issue text.