expose SystemDependency and BuiltinDependency as toplevel classes

mesonbuild.dependencies.__init__ exposes configtool, pkgconfig, cmake
and more in __init__.py, so there's no reason we should be tying
SystemDependency to the internal organization implementation of the
subpackage!

In the 2nd previous commit it took quite some effort to figure out that
the python module "does not exist" because of import errors while
refactoring something completely different.

mesonbuild/dependencies/__init__.py

@@ -16,7 +16,9 @@ from .boost import BoostDependency
 from .cuda import CudaDependency
 from .hdf5 import hdf5_factory
 from .base import Dependency, InternalDependency, ExternalDependency, NotFoundDependency
-from .base import ExternalLibrary, DependencyException, DependencyMethods
+from .base import (
+        ExternalLibrary, DependencyException, DependencyMethods,
+        BuiltinDependency, SystemDependency)
 from .cmake import CMakeDependency
 from .configtool import ConfigToolDependency
 from .dub import DubDependency
@@ -43,6 +45,8 @@ __all__ = [
     'Dependency',
     'InternalDependency',
     'ExternalDependency',
+    'SystemDependency',
+    'BuiltinDependency',
     'NotFoundDependency',
     'ExternalLibrary',
     'DependencyException',

mesonbuild/modules/python.py

@@ -30,8 +30,7 @@ from ..interpreter import ExternalProgramHolder, extract_required_kwarg, permitt
 from ..build import known_shmod_kwargs
 from .. import mlog
 from ..environment import detect_cpu_family
-from ..dependencies import DependencyMethods, PkgConfigDependency, NotFoundDependency
+from ..dependencies import DependencyMethods, PkgConfigDependency, NotFoundDependency, SystemDependency
-from ..dependencies.base import SystemDependency
 from ..programs import ExternalProgram, NonExistingExternalProgram
 
 mod_kwargs = {'subdir'}