Fix workspace manifest loading

[bnjbvr]

This fixes two things:

• first, we looked at the wrong manifest when checking for a [workspace] property: we need to look at the workspace manifest, to the child one.
• second: there is an "API bug" in cargo_toml, that makes it impossible to properly use from_path_with_metadata (unless I'm mistaken! see full analysis), so we need to use other methods to get the content, with a slightly different order. That's fine, it's quite internal and shouldn't change much over time.

@Veetaha Would you be so kind as to try this branch, please, to confirm whether this fixes the issue you were running into? At least for me, it does fix it where I saw it (internal repositories).

[bnjbvr]

taking a leap of faith as it really works locally :)