Improve NamespaceRegistry

[theofidry]

• Make the NamespaceRegistry code more readable
• Leverage str_contains over strpos to make the code clearer
• Add more tests to ensure we also test against sub-namespaces
• Add more tests to ensure we also test against dirty-symbols (i.e. with a leading backslash)
• Improve performances if global namespace is registered (it will skip all names + regexes checks)

A minor changes occurs in the spec were previously the class namespace was not detected as excluded hence could end up cleaned up. This is a very narrow edge case where the string literal is already prefixed: the only case affected is probably the specs themselves.