Optional dependency that is not enabled still considered as part of the license checks

[sdroege]

To Reproduce
Steps to reproduce the behavior:

1. Go to https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/tree/master/gstreamer
2. Check the Cargo.toml and notice the gstreamer-rs-lgpl-docs crate as an optional dependency hidden behind a feature flag
3. Try running cargo deny check in the whole cargo workspace after removing the licenses.exceptions from deny.toml
4. Notice that cargo deny complains about there being a copyleft license involved, and that LGPL-2.0 is not a allowed license

Expected behavior

No problematic license would be found as the feature flag is not enabled by anything in the repository.

[Jake-Shadle]

This is because we gather metadata with --all-features enabled, I will add support for the normal cargo feature-related options so that you can specify which ones you want enabled when checking, as long as you specify a specific crate's manifest path and not a workspace.

[sdroege]

Thanks!