Is it possible to change "msg" and "srv" folder position in a package ?

edit

Those locations are not configurable.

Unfortunately, the assumptions about where the message/service source files are and where the generated code goes are embedded in both rosbuild and in the code-generation logic in all the client libraries. The msg_gen and srv_gen locations are further embedded in rospack, which looks for a file called generated in each of those directories to decide whether to add msg_gen/cpp/include and srv_gen/cpp/include to the compiler include path.

While all those assumptions in code could be made configurable, it would be a large, and largely untested, change.

If relocating those files is important to you, I recommend requesting an enhancement in rosbuild2. Be sure to describe your use case thoroughly so that we can understand what's needed, and judge the tradeoffs involved.