dependencies: Fix type of dependency_factory decorator

There was both a straight up bug in the type signature (the return type
is List[Callable[[], Dependency]] not List[Type[Dependency]]), and in
the way the arguments are assembled. Typing is pretty limited in it's
ability to express decorators, so the best mypy can do is check the
return types (I think). I've done what the docs suggest and it's stopped
complaining.

mesonbuild/dependencies/base.py

@@ -2498,10 +2498,11 @@ def strip_system_libdirs(environment, for_machine: MachineChoice, link_args):
     return [l for l in link_args if l not in exclude]
 
 
-def process_method_kw(possible: T.List[DependencyMethods], kwargs) -> T.List[DependencyMethods]:
-    method = kwargs.get('method', 'auto')
+def process_method_kw(possible: T.Iterable[DependencyMethods], kwargs) -> T.List[DependencyMethods]:
+    method = kwargs.get('method', 'auto')  # type: T.Union[DependencyMethods, str]
     if isinstance(method, DependencyMethods):
-        return method
+        return [method]
+    # TODO: try/except?
     if method not in [e.value for e in DependencyMethods]:
         raise DependencyException('method {!r} is invalid'.format(method))
     method = DependencyMethods(method)
@@ -2519,26 +2520,23 @@ def process_method_kw(possible: T.List[DependencyMethods], kwargs) -> T.List[Dep
     # Set the detection method. If the method is set to auto, use any available method.
     # If method is set to a specific string, allow only that detection method.
     if method == DependencyMethods.AUTO:
-        methods = possible
+        methods = list(possible)
     elif method in possible:
         methods = [method]
     else:
         raise DependencyException(
             'Unsupported detection method: {}, allowed methods are {}'.format(
                 method.value,
-                mlog.format_list([x.value for x in [DependencyMethods.AUTO] + possible])))
+                mlog.format_list([x.value for x in [DependencyMethods.AUTO] + list(possible)])))
 
     return methods
 
 
 if T.TYPE_CHECKING:
-    FactoryType = T.Callable[[Environment, MachineChoice, T.Dict[str, T.Any]],
-                             T.List['DependencyType']]
-    FullFactoryType = T.Callable[[Environment, MachineChoice, T.Dict[str, T.Any], T.Set[DependencyMethods]],
-                                 T.List['DependencyType']]
+    FactoryType = T.TypeVar('FactoryType', bound=T.Callable[..., T.List[T.Callable[[], 'Dependency']]])
 
 
-def factory_methods(methods: T.Set[DependencyMethods]) -> 'FactoryType':
+def factory_methods(methods: T.Set[DependencyMethods]) -> T.Callable[['FactoryType'], 'FactoryType']:
     """Decorator for handling methods for dependency factory functions.
 
     This helps to make factory functions self documenting
@@ -2547,13 +2545,13 @@ def factory_methods(methods: T.Set[DependencyMethods]) -> 'FactoryType':
     >>>     pass
     """
 
-    def inner(func: 'FullFactoryType') -> 'FactoryType':
+    def inner(func: 'FactoryType') -> 'FactoryType':
 
         @functools.wraps(func)
-        def wrapped(env: Environment, for_machine: MachineChoice, kwargs: T.Dict[str, T.Any]) -> T.List['DependencyType']:
+        def wrapped(env: Environment, for_machine: MachineChoice, kwargs: T.Dict[str, T.Any]) -> T.List[T.Callable[[], 'Dependency']]:
             return func(env, for_machine, kwargs, process_method_kw(methods, kwargs))
 
-        return wrapped
+        return T.cast('FactoryType', wrapped)
 
     return inner