[TASK] Allow registering namespaces multiple times unless there are conflicts
Currently the ``TemplateParser`` throws an exception if the same
namespace identifier is registered multiple times.
With this change this exception is skipped if the repeated
registration does not pose conflicts, i.e. points to the same
PHP namespace as previous mappings.
This is a follow-up to I965cb54c3125f80e7a5ae46ede72ee9027ed006e that
registers all package keys as Fluid namespaces.
Due to some unrelated bug ``PackageManager::getActivePackages()`` can
return the same package key twice leading to an exception:
"#1224241246: Namespace identifier "xyz" is already registered.".