Add support for loading plugins via command line (fixes #15446)

`rustc something.rs -L folder_with_plugin_dylib/ -Z extra-plugins=foo` works via this

My way of testing this is by cloning https://github.com/Manishearth/rust-clippy, `cargo build`ing it, and then running `rustc examples/box_vec.rs -L target/ -Z extra-plugins=rust_clippy` after editing out the `exern crate rust_clippy` from `box_vec.rs`

r? @huonw