Expand 'goto definition' to more use components

[kellytk]

It would be helpful for learning and code exploration if 'goto definition' support was added to all use components. It appears that only the last component, when not *, supports 'goto definition.'

For example, with use actix::prelude::*; 'goto definition' is not supported at all. With use std::fs, 'goto definition' is supported only for fs.

[kellytk]

Moving issue to rust-lang/vscode-rust#418.