Language specific package support

[benradf]

Is your feature request related to a problem? Please describe.
When a Nix+Bazel project needs to depend on language specific third-party packages, it is not always clear how to do this. For example, depending on Rust crates, NPM modules, Hackage packages, etc.

For some languages there is already good support. In rules_haskell you can provide system libs to stack_snapshot via the extra_deps attribute. For others there is no support currently, e.g. npm_install and pip_install will use a global interpreter and will build native lib bindings outside the sandbox.

Describe the solution you'd like
Ultimately it would be desirable to provide good language specific package support for all the toolchains in rules_nixpkgs. However that will likely take some time and effort to achieve, so in the meantime it would be helpful to provide best practice examples to guide users (like the Rust openssl dependency example).