Improve libraries imports

This MR removes libraries importation from rustc (starting with rustc_) and uses the crate wrapper rr_rustc_interface.

As this MR changes imports, closed work has also been done:

• Use absolute paths (use crate::) instead of relative paths (use self::/use super::); relative paths were found less frequently than absolute paths when both choices were possible.
• Remove renamed imports applied to library names (use ... as ...)
• Remove deprecated import syntax crate (extern crate ...); this is done automatically by the compiler

rr_frontend/attribute_parse/src/lib.rs

 #![feature(rustc_private)]
 
-extern crate rustc_ast;
-extern crate rustc_driver;
-extern crate rustc_span;
-
 pub mod parse;