Plugin api named_type() is ambiguous and inconsistent and broken

The plugin API defines two different interfaces (TypeAnalyzerPluginInterface and SemanticAnalyzerPluginInterface) that each define a method named_type(). These look almost identical, but looks are deceptive.

First, the specified signatures differ subtly:

• The second argument to SemanticAnalyzerPluginInterface.named_type() is optional, while for TypeAnalyzerPluginInterface.named_type() is is mandatory.
• Also the first argument name differs (qualified_name vs. plain name).

Second, the implementations differ:

• The semantic analyzer (in semanal.py) starts the lookup in the local namespace, then globals, then builtins.
• The type checker (in checker.py) has two different strategies: if there's no dot in the name, it starts in globals, then builtins; but if there's a dot, it starts in the table of modules (i.e. the current scope doesn't enter into it).

Third, the type checker's named_type() implementation doesn't actually take a list of type arguments at all -- it's a method with only one argument, the name to look up. It always fills in all type parameters with Any. (It looks like the method we want here is named_generic_type().)

Fourth, apparently mypy's own type checking doesn't catch the discrepancy between the implemented spec (only a name) and the interface spec (name + args).

Start by comparing TypeAnalyzerPluginInterface.named_type() and SemanticAnalyzerPluginInterface.named_type() with the implementations in semanal.py and checker.py. Inspect named_generic_type() and the type-checking setup to understand why the signature discrepancy is not caught. Done means the plugin interfaces and implementations have a consistent, agreed API and the mismatch is covered by type checking or tests.