Identify proc_macro dependencies

[jethrogb]

When cross-compiling, it's useful to know which crates are compiled for the host and which for the target. The [build-dependencies] and [dev-dependencies] sections are already useful for filtering some of this information. However, proc_macro crates are shown as regular dependencies.

[sfackler]

Ah yeah, seems like it'd make sense to split out dependencies rooted at a plugin = true crate to [plugin-dependencies] or something like that.