Separate translators into "from MLIR" and "to MLIR".

Translations performed by mlir-translate only have MLIR on one end.
MLIR-to-MLIR conversions (including dialect changes) should be treated as
passes and run by mlir-opt.  Individual translations should not care about
reading or writing MLIR and should work on in-memory representation of MLIR
modules instead.  Split the TranslateFunction interface and the translate
registry into two parts: "from MLIR" and "to MLIR".

Update mlir-translate to handle both registries together by wrapping
translation functions into source-to-source convresions.  Remove MLIR parsing
and writing from individual translations and make them operate on Modules
instead.  This removes the need for individual translators to include
tools/mlir-translate/mlir-translate.h, which can now be safely removed.

Remove mlir-to-mlir translation that only existed as a registration example and
use mlir-opt instead for tests.

PiperOrigin-RevId: 222398707