apis: west.manifest.Manifest.projects should not include the manifest itself

Trying to treat the manifest repository as an ordinary Project was doomed to failure; they’re simply too different. We should instead move the valid state that’s in ManifestProject today to Manifest attributes, removing MANIFEST_PROJECT_INDEX and the fact that projects[MANIFEST_PROJECT_INDEX] contains the manifest.

This will require a bit of thought into how west list should behave, though. Maybe we need to resurrect west list --all to include manifest project data like {path} and {west_commands}? And perhaps that should abort if you west list --all -f {url} or try to access some other attribute that’s not defined for the manifest repository.

[mbolivar]

For sure, west list should continue to contain the manifest repository. It’s just the internal API I want to change.

[tejlmand]

⚠️ In case you start doing some work on this, remember that Zephyr modules uses west list to obtain a list of ALL projects, as those are candidates for being a Zephyr module: https://github.com/zephyrproject-rtos/zephyr/blob/51fc680ba225c2df5f3fca9064839b825a9c0511/scripts/zephyr_module.py#L177

and in some cases the manifest project is ALSO a Zephyr module, and it would be bad to have west break backward compatibility to projects based on older Zephyr revisions. I would prefer a new west to also work if i checkout an old version of NCS, as example.