Dlang uses both integer version "levels" and arbitrary string
identifiers, and we support both, but don't mention it in the docs.
Also update a test case to pass one via declare_dependency. We already
test this kwarg for build_target.
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