Naming conventions for targets

[mgrrx]

Some targets are prefixed with cc or py, for some others it's not consistent. I'm happy to provide a PR. What would you prefer?

[mvukov]

Here is some background:

• ros_interfacelibrary groups/provides ROS IDL, then cc and py_ros_interface_library are C++ and Python variants (doing code generation internally).
• cc(binary|library) are built in rules for C++ targets. ros variants are ROS specific macros.

In this sense naming is consistent IMO, though I understand confusion. Perhaps docs can be updated. WDYT?