cmd/pkgserver: use IR cache for ident

This removes requirement to open full cache.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/pkgserver/index_test.go

@@ -0,0 +1,14 @@
+package main
+
+import "testing"
+
+// newIndex returns the address of a newly populated packageIndex.
+func newIndex(t *testing.T) *packageIndex {
+	t.Helper()
+
+	var index packageIndex
+	if err := index.populate(nil); err != nil {
+		t.Fatalf("populate: error = %v", err)
+	}
+	return &index
+}