refactor la backends discovery/usage

* common.so -> _common.so, to allow for magic in __init__.py
* create module infrastructure in build tree by symlink during cmake

reduces spurious re-runs by only updatin gthe builder defition
when it actually changed

- now shows which new/revomed tests trigger rebuilding
- no longer limited to building with ninja

This reverts commit b50a7d9b.
It was actually missing in dune-gdt, plus typo ridden too.