Commands cannot always load projects with advanced grammar

### What needs to get done

A command cannot parse a project with advanced grammar if no build-for or platform is provided.

Source: https://github.com/canonical/snapcraft/pull/4630#issuecomment-1988210684

As discussed as a team, this may be solved by loading only some items of the project.

### Why it needs to get done

To allow applications to apply grammar without requiring the `build_for` or `platform` arguments. See an example of the problem in snapcraft's [ google:ubuntu-24.04-64:tests/spread/core24/grammar:amd64](https://github.com/canonical/snapcraft/actions/runs/8209947752/job/22456602084#step:4:376).

Start with the linked Snapcraft pull-request discussion and the referenced spread job, tests/spread/core24/grammar:amd64, to reproduce the failure when build_for or platform is absent. Trace project loading for advanced grammar and confirm that commands can load the applicable project items without those arguments.

Written by the indexing model from the issue text.