Tweaking the way the warning is displayed

[mxgrey]

Following the upstream discussion I started playing around with the way the warning gets displayed.

Here's where I settled after my own experimentation:

• Pad the warning with lots of newlines so the warning doesn't get lost in the noise of the header
• Push the $ to the right with a space to make it stand out a little better
• Use a global variable so that the warning is only printed once, no matter how many ament-cargo packages are in the workspace

The resulting terminal output looks like this:

I'd prefer if it could be highlighted in yellow or red, but I think this is visible enough that users won't easily overlook it.