Use `cargo_metadata` to get manifest data

[oeb25]

This commit uses the cargo_metadata crate to derive information about the crate, as opposed to only parsing Cargo.toml.

In particular, this helps when fields in the manifest are inherited from the workspace, for example if version = { workspace = true }.

One thing cargo_metadata does not provide, is badges, so these are still parsed from Cargo.toml.

The error messages also changed as a result of propagating the errors from cargo_metadata, and thus some of the tests had to be altered.