[mlir] Make Python bindings installable.

* Links against libMLIR.so if the project is built for DYLIBs.
* Puts things in the right place in build and install time python/ trees so that RPaths line up.
* Adds install actions to install both the extension and sources.
* Copies py source files to the build directory to match (consistent layout between build/install time and one place to point a PYTHONPATH for tests and interactive use).
* Finally, "import mlir" from an installed LLVM just works.

Differential Revision: https://reviews.llvm.org/D89167