[mlir][python] Tweaks to make python extensions packagable/distributable.

* Works in tandem with prototype packaging scripts here: https://github.com/stellaraccident/mlir-py-release
* The `mlir` top-level now differentiates between in-tree builds where all packages are co-located and distribution mode where all native components are under a top-level `_mlir_libs` package.
* Also fixes the generated dialect python installation again. Hopefully the last tweak.
* With this, I am able to install and generate archives with the above setup script on Linux. Archive size=31M with just host codegen and headers/shared-libraries. Will need more linker tweaks when wiring up the next dependent project.

Differential Revision: https://reviews.llvm.org/D93936