[mlir][bufferize][NFC] Implement BufferizableOpInterface on bufferization ops directly

No longer go through an external model. Also put BufferizableOpInterface into the same build target as the BufferizationDialect. This allows for some code reuse between BufferizationOps canonicalizers and BufferizableOpInterface implementations.

Differential Revision: https://reviews.llvm.org/D117987