I broke `jj debug reindex` in a27da7d8d5. From that commit, we no
longer delete the pointer to the old index, so nothing happens when we
reload the index. This commit fixes that, and also makes the command
error out if run on a repo with a non-default index type.
This is yet another step towards making the index pluggable. The
`IndexStore` trait seems reasonable after this commit. There's still a
lot of work to remove `IndexPosition` from the `Index` trait.
I didn't make `ReadonlyIndex` extend `Index` because it needed an
`as_index()` to convert to `&dyn Index` trait object
anyway. Separating the types also gives us flexibility to implement
the two traits on different types.
Not all index implementations may want to store the readonly index
implementation in an Arc. Exposing the Arc in the interface is also
problematic because `Arc<IndexImpl>` cannot be cast to `Arc<dyn
Index>`.
These two files are closely related, and `Index` and `IndexStore` are
expected to be customized together, so it seems better to keep them in
a single file.
