Perform analysis over virtual manifest

[nasifimtiazohi]

For a workspace that only declares member packages, Cargo geiger fails to execute with below message

manifest path "[path_to_workspace]/Cargo.toml" is a virtual manifest, but this command requires running against an actual package in this workspace

However, this functionality would help for generating a single report for the whole workspace with possible de-duplication of many analysis.

[nasifimtiazohi]

For anyone looking, creating a super package replicating the workspace's dependency graph with this crate can be a workaround - https://docs.rs/depdive/0.1.0/depdive/super_toml/index.html