rust: Expose import names in completions for modules and functions (#14490)

Release Notes:

- Improved accuracy of completion lists for Rust functions and modules.

crates/languages/Cargo.toml

@@ -58,6 +58,6 @@ util.workspace = true
 
 [dev-dependencies]
 text.workspace = true
-theme.workspace = true
+theme = { workspace = true, features = ["test-support"] }
 unindent.workspace = true
 workspace = { workspace = true, features = ["test-support"] }