Auto merge of rust-lang#16258 - Veykril:import-map-fix, r=Veykril

internal: Rewrite `ImportMap::search_dependencies`

Second attempt, this time with a lot of perf improvements

Fixes rust-lang/rust-analyzer#16080
Fixes rust-lang/rust-analyzer#16074
Fixes rust-lang/rust-analyzer#15845

The main bottleneck for flyimport completions are now import path calculation and completion item rendering.

Author: bors