Proper support for import aliases and attributes on imports

[Veykril]

We currently don't really support aliasing imports at all (aliases usually just break things or make them not work at all), likewise we kind of ignore most attributes on imports, like #[doc(hidden)] and similar things. We should make these work, though it's not clear how to tackle this.

some issues depending on this:

• Listing symbols does not see aliases #7938
• Trying to rename "use alias" in VSCode messes up with files from other crates #5198
• Import/qualify trait Deref suggesting lazy_static::__Deref instead of std::ops::Deref #9197
• Prefer importing items from the crate they are defined in, rather than a re-export. #11698
• Show rustdoc of proc macros #14772
• Renaming use alias renames the aliased symbol instead #5671
• Public re-exports do not appear in code completion #17042