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

[Veykril]

So, what's roughly missing here to finish this issue up is the following:

Our Name{Ref} "classification" needs to be changed to accomodate for imports and extern crate declarations. I am unsure of the interface we want here, we might just want to have two APIs, one that returns Definitions as previously and another that returns Definitions and the Use/ExternCrateDecl that the Definition is coming from (unless it is defined in this scope). The relevant part for this is this file here https://github.com/rust-lang/rust-analyzer/blob/master/crates/ide-db/src/defs.rs

Then we just need to adjust all the IDE features that would want to care about the imports in that regard. For that we need to implement a way to step through the import chain. We already have a function to fully re-resolve an import here

rust-analyzer/crates/hir-def/src/item_scope.rs

Line 162 in 91237a5

pub fn fully_resolve_import(&self, db: &dyn DefDatabase, mut import: ImportId) -> PerNs {

Stepping through resolutions requires more effort though due to proper namespace handling and exposing a decent API for that in the IDE layer. Part of the task would be figuring that out.