snapcraft-specific code in `launched_environment()` is removed from the craft-providers interface. - [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 `make lint`? - [X] Have you successfully run `pytest tests/unit`? ----- ### Overview Removes snapcraft-specific logic from `launched_environment()`. This function is now matches the implementation in rockcraft ([lxd](https://github.com/canonical/rockcraft/blob/97306bc3d93c8a41f18432b1dadf4569c7f28854/rockcraft/providers/_lxd.py#L99) and [multipass](https://github.com/canonical/rockcraft/blob/97306bc3d93c8a41f18432b1dadf4569c7f28854/rockcraft/providers/_multipass.py#L89)) This is the most complex PR of the series, due to the translation of bases. The base translation was added in [this PR](https://github.com/canonical/rockcraft/pull/85) to rockcraft. ### Base Translation #### Background *craft applications have a concept of a "base". This base is translated a `craft-providers` base and is passed to `craft-providers` so it can choose which base image (OS and release) to use for the instance. #### Problem Currently, each application translates its own base inside `_lxd.py` and `_multipass.py`. This application-specific code needs to be removed from these files. #### Solution To support this, I created a list of what bases craft-providers can accept. Right now, `18.04`, `20.04`, and `22.04` are supported. Other OSs and releases can be added in the future without breaking backwards compatibility. The image below shows how an application can translates its base to a `craft-providers` base. It also shows `craft-providers` translating it to the specific name needed for LXD and Multipass to download the base image.  ### Note about the changeset This PR shows ` 10 files changed, 768 insertions(+), 274 deletions(-)`, but it's much smaller. It's really `8 files changed, 164 insertions(+), 122 deletions(-)`. This is because I copied over rockcraft's files `tests/unit/providers/test_{lxd,multipass}.py`. These files have already been reviewed and are going to be migrated to `craft-providers` later this week. Thus, these unit test files probably don't need intense scrutiny. (CRAFT-1359)