[mlir] Use public PybindAdaptors in Linalg dialect bindings

Previously, the Python bindings for the Linalg dialect relied on the internal
implementation of core bindings. Most of that functionality was moved, and the
remaining one does not need access to the implementation: it used to accept a
dialect pointer as argument, but it can always be extracted from the operation
that it also accepts; operations are available through PybindAdaptors in an
opaque way. Change the bindings in that direction.

This enables the decoupling of the Linalg dialect Python extension from the
core IR Python extension.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D116649