Add sniff to enforce importing a namespace not a namespaced function/constant

[jrfnl]

Discussed on Slack with @herregroen.

For namespaced functions and constants, disallow importing these via use func/ use const.
Instead, the namespace should be imported and the function name/constant name should be qualified with the namespace name.

Enforce:

use Package\Level;

Level\function_name();

Disallow:

use func Package\Level\function_name;

function_name();

[jrfnl]

I'm going to close this issue as "fixed".

Importing functions/constants is now disallowed. While it is not enforced that the namespace is imported, that is realistically, the only other alternative.