*Warning: this is a large PR, larger than I'd like - but it snowballed over the last two days given the coupling between things... For discussion purposes anyhow...* Instead of allowing support for multiple apt instances, we consolidate to the system's configuration. As build-environments are instantiated for builds, we do not have the prior concerns of impacting the user's system (in destructive mode). Even in destructive mode, we shouldn't be causing damage. - Use a single instance of the apt cache, which should improve performance due to the penalty of recreating caches in numerous points. It also simplifies usage without the requirement of copying/sharing instances for what is (now) a system-wide resource. - Remove old scheme for gathering stage packages, and simply do a scan ourselves for the packages and their dependencies. Then fetch them using python-apt's fetch_binary(). - Merge in the manifest.txt with a pre-processed default list of the filtered packages. Future work will attempt to use the manifest shipped as part of the core snap. - Rework of the ROS plugins to remove their current usage of Ubuntu() and replace it with a simpler set of APIs to simply install the required repository and keyrings. TODO: - squelch python-apt's debug log - fixup tests Signed-off-by: Chris Patterson <chris.patterson@canonical.com> - [ ] Have you followed the [guidelines for contributing](https://github.com/snapcore/snapcraft/blob/master/CONTRIBUTING.md)? - [ ] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? - [ ] Have you successfully run `./runtests.sh static`? - [ ] Have you successfully run `./runtests.sh tests/unit`? -----