Rename rustdoc::html::render::cache to search_index

The old name wasn't very clear, while the new one makes it clear that
this is the code responsible for creating the search index.

src/librustdoc/html/render/context.rs

@@ -13,8 +13,8 @@ use rustc_span::edition::Edition;
 use rustc_span::source_map::FileName;
 use rustc_span::symbol::sym;
 
-use super::cache::{build_index, ExternalLocation};
 use super::print_item::{full_path, item_path, print_item};
+use super::search_index::{build_index, ExternalLocation};
 use super::templates;
 use super::write_shared::write_shared;
 use super::{

src/librustdoc/html/render/mod.rs

@@ -23,7 +23,7 @@
 //! These threads are not parallelized (they haven't been a bottleneck yet), and
 //! both occur before the crate is rendered.
 
-crate mod cache;
+crate mod search_index;
 
 #[cfg(test)]
 mod tests;