mconf: Don't change the type of coredata.deps

Which is an OrderedDict, but mconf changes to a plain dict. This will
work fine with Python 3.7+, but not with 3.5 and 3.6

mesonbuild/mconf.py

@@ -63,7 +63,7 @@ class Conf:
             raise ConfException('Directory {} is neither a Meson build directory nor a project source directory.'.format(build_dir))
 
     def clear_cache(self):
-        self.coredata.deps = {}
+        self.coredata.deps.clear()
 
     def set_options(self, options):
         self.coredata.set_options(options)