2381: Add proc-macro crate type handling r=JasperDeSutter a=JasperDeSutter
Resolves the libproc_macro crate in crates that are the proc-macro type.
This doesn't seem the ideal implementation though, since the compiler still requires you to write `extern crate proc_macro;` (even in 2018 edition).
Co-authored-by: JasperDeSutter <jasper.desutter@gmail.com>
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