Move hack_dependencies_not_implemented into a class to make the file
structure easier to understand and reduce the visibility of the
_implemented_dependencies cache. Rename it because it's no longer a
temporary hack (originally intended to work around the fact that not all
PSA_WANT symbols were implemented), it's now a way to detect test cases for
cryptographic mechanisms that are declared but not implemented.
Internal refactoring only. No behavior change.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>