refactor: default to ros-<distro>- prefixed names.

[ruben-arts]

Closes #355

This PR inverts the logic of the default package name.
Assuming this package.xml:

    <buildtool_depend>catkin</buildtool_depend>
    <depend>ros_package</depend>

This would make ros_package a conda dependency. We've decided that is not the right approach.
This PR changes that assumption to ros-<distro>-ros-package and only if defined otherwise in the package_map it will use the defined name in the map.

ps. I adjusted the tests so they also run on macOS.