Describe the problem you are trying to solve
I'm trying to use cargo vendor to easily share files/projects and be able to perform remote builds.
Some of my project dependencies are private (hosted in private git repos) while others are in public repositories, e.g. crates.io
Running cargo vendor downloads everything into vendor folder by default and produces something like the following config.toml

[source.crates-io]
replace-with = "vendored-sources"

[source."https://bitbucket.private_repo_name.git"]
git = "https://bitbucket.private_repo_name.git"
tag = "v0.0.205"
replace-with = "vendored-sources"

[source."https://bitbucket.private_repo_name2.git"]
git = "https://bitbucket.private_repo_name2.git"
replace-with = "vendored-sources"

[source.vendored-sources]
directory = "vendor"

From a working perspective this approach works, but is not great as the vendor folder becomes slower to synchronise and the remote server I'm sending files to can already access crates.io (but not https://bitbucket.private_repo_name.git or https://bitbucket.private_repo_name2.git which is why I'm vendoring)

Describe the solution you'd like
It would be great to tell cargo vendor what to vendor and ignore everything else, or tell it what not to vendor and vendor everything else.
The vendor directory would only contain specific crates (private in my case), and the config.toml would only have "source." entries for those vendores crates

Notes
I'd be happy to contribute if this feature sounds relevant and generic enough.