Removed genmsg-in-package, changed force-genmsg to genmsg, fixed bugs

I think there should be just one way of generating messages in rosgo, and I think the way which was called "rosgo_force_generate_messages" is better.  It generated the messages in the devel directory, not in the source.  Generating anything in source directories is a problem because it makes version control difficult.

rosgo_force_generate_messages was giving an error when I included a message more than once (like once from one catkin package and once from another).  I added a conditional to it so it only generates each message package once and doesn't give an error.

I changed the name to remove the word "force" since at this point it seems like just a regular message generation.

Since rosgo is divided up over 6 packages, there will be other pull requests following this to convert the other packages to this new cmake code.  I recommend merging all 6 rosgo repositories into a single one.  It will be much easier to manage changes.  I may do that in another pull request. :)

Did you ever get that t-shirt I sent?  Thanks again for doing this rosgo work.