This PR is a follow-up on #3536 and essentially fixes the same issues for ROS 2 plugins v2. This PR most notably, - changes the colcon install path from `$SNAPCRAFT_PART_INSTALL` to `$SNAPCRAFT_PART_INSTALL/opt/ros/snap` for a cleaner separation of ROS 2 workspaces (similar to what's in the v1 plugin). - looks for ROS 2 packages in `$SNAPCRAFT_PART_INSTALL/opt/ros/snap` in case the workspace exists (e.g. provided by a stage-snap) during the build step - does not `rosdep resolve` packages that are available locally in `$SNAPCRAFT_PART_INSTALL` during stage step. Such locally available package may be part of the built sources or provided by a stage-snap. Furthermore, it fixes the following current issue; if the source contains packages `foo` & `bar` - two packages that are not released in the rosdistro database - where `foo` depends on `bar`, staging fails as it tries to `rosdep resolve` the `bar` key (which does not exists since it is not released). With this fix, it finds `bar` locally and thus does not try to resolve it. This issue is fixed by deleting the `${SNAPCRAFT_PART_INSTALL}/opt/ros/snap/COLCON_IGNORE` marker file - if it exists - right after the actual build command. This marker file prevents catkin from crawling the directory while looking for packages during staging. - adds `test-snapcraft-fake-ros2-package-core20` snap and `colcon-stage-snaps` snap/spread test to test the above bullet points. Note: - I own the `test-snapcraft-fake-ros2-package-core20` snap that's used in the new spread test, I imagine we should transfer ownership. - [x] Have you followed the [guidelines for contributing](https://github.com/snapcore/snapcraft/blob/master/CONTRIBUTING.md)? - [x] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? - [x] Have you successfully run `./runtests.sh static`? - [x] Have you successfully run `./runtests.sh tests/unit`? -----