[mlir][bazel] Export headers either from :Transforms or :TransformUtils (#86819)

Split them according to their implementation.

Ideally, header files should be used by only one target, but this is
hard because CMake is less strict with headers (no layering check). But
even with bazel, headers should only be exported once in the `hdrs`
attribute. Other targets may use them in the `srcs` attribute to avoid
circular dependencies.

Author: chsigg