[mlir][sparse] moving kInvalidId into "detail" namespace

In the next few commits I will be converting the various Merger identifier typedefs into newtypes; and once that's done, the `kInvalidId` constant will only be used internally and therefore does not need to be part of the public `mlir::sparse_tensor` namespace.

Depends On D146673

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D146674