Each tech has a different metal stackup, so user projects get a different top routing metal layer. tt-support-tools now handles this instead of the user having to worry about this when switching PDKs.
Make the codebase more PDK-agnostic by moving tech-specific code to `tech.py` and tech specific files (e.g. def files, tile_sizes.yaml) to `tech/<pdkname>` subdirectory.