Allow crate authors to control completion of their things

Via the new `#[rust_analyzer::completions(...)]` attribute.

Also fix a bug with existing settings for that where the paths wouldn't resolve correctly.

crates/intern/src/symbol/symbols.rs

@@ -521,4 +521,8 @@ define_symbols! {
     win64,
     array,
     boxed_slice,
+    completions,
+    ignore_flyimport,
+    ignore_flyimport_methods,
+    ignore_methods,
 }