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