feat: add package specific dependencies and errors

[baszalmstra]

Builds on #2544 .

This PR adds dependencies to either workspace or package targets based on the rest of the content of the manifest.

E.g.

[project]

[host-dependencies]

[dependencies]

Adds both the host-dependencies and the dependencies to the default target of the default feature of the workspace/project. But:

[workspace]
preview = ["pixi-build"]

[dependencies]

[package]

[host-dependencies]

Adds the host-dependencies to the default target of the package manifest instead.

I also added loads of corner error cases with helpful snippets and help messages to clarify what happens when enabling the preview feature (see the pushed snippets).

Build backends should read the package section of the manifest and disregard the workspace/project section. But thats for follow-up PRs to the backends.