Improve content management for launch packages

[ncoghlan]

Launch packages currently ship everything inside the specified folder. This means things like README.md files that shouldn't be deployed have to be kept outside the package folder they relate to.

It would be useful to support at least the choice between path/to/launch_package/** and path/to/launch_package/*.py when deciding which launch package files to copy to the build folder for deployment (any changes to the filter that affected the included files would show up in the input metadata as a change to the directory hash)