Fix possible double import of cubes For a new-style cube, if we import "cubicweb_<name>" and then import "cubes.<name>", the cube will be imported twice. cubes.<name> and then cubicweb_name is ok though... When using pyramid, we try to find which cube define a "includeme" by importing them with the name "cubes.<name>", so we (possibly ?) double import all new-style cube. This case may also occur in tests with PyramidCWTest. Touching the import loader to fix this scares me, so let's fix this by testing "cubicweb_<name>" *before* "cubes.name" (for old style cubes, importing cubicweb_<name> raise ImportError).