Split curl dependency to avoid dev package at runtime

[cottsay]

The rosdep keys were (recently) added in ros/rosdistro#31301.

From what I can tell, the curl headers are not needed by packages building against this package, so libcurl-dev doesn't need to be a build_export_depend.

[clalancette]

This looks reasonable to me. We should also probably port this over to the ros2 branch as well.

[cottsay]

We should also probably port this over to the ros2 branch as well.

In ROS 2, we're using libcurl_vendor, so this change doesn't apply there.

[cottsay]

I tested this change by building up to resource_retriever, uninstalling libcurl4-openssl-dev, and then continuing the build up to rviz. The build was successful and ldd rviz shows that resource_retriever is linked correctly against libcurl.so.4, which is still installed. So this appears to work as intended.