Inside of mesonbuild.ast.* we can and should import from .foobar, rather
than importing from .__init__'s re-exported version of that object.
Failing to do so results in an extremely brittle codebase where simply
changing the order of lines in __init__.py can result in ImportError.
D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082