Handle cargo workspaces in git repositories

[Flamefire]

Crates downloaded from crates.io can just be put into the vendored-sources directory.
However a referenced git repository might contain multiple crates in a so-called "workspace".
If we put such a workspace into the vendor folder, cargo fails with

found a virtual manifest at [...]Cargo.toml instead of a package manifest

This solution uses a separate folder for sources downloaded from git repositories and checks them for workspaces.
If they are "regular", i.e. don't have a workspace, they are put to the vendored sources from crates.io.
Otherwise they are left in the git-vendor directory.

Additionally refine the config.toml entry for sources vendored from git. This allows to override a specific git URL and revision with the path to the extracted repository.
Otherwise we would need entries for each crate in workspaces but then cargo tries to verify the revision and fails with

Unable to update https://github.com/[...]?rev=[...]
Caused by: can't checkout from 'https://github.com/[...]': you are in the offline mode (--offline)

I also have a Stack Overflow question on that in case there is a better solution: https://stackoverflow.com/questions/79065892/how-to-patch-a-cargo-git-dependency-that-contains-a-workspace