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).
