stage2: fix panic when the dependency is missing

Author: ri-char

src/Sema.zig

@@ -10379,7 +10379,9 @@ fn zirImport(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.
                     }
                 }
                 unreachable;
-            } else unreachable;
+            } else {
+                return sema.fail(block, operand_src, "no package named '{s}' available", .{operand});
+            };
             return sema.fail(block, operand_src, "no package named '{s}' available within package '{s}'", .{ operand, parent });
         },
         else => {

src/main.zig

@@ -2750,8 +2750,22 @@ fn buildOutputType(
 
     // Transfer packages added with --pkg-begin/--pkg-end to the root package
     if (main_pkg) |pkg| {
+        var it = pkg_tree_root.table.valueIterator();
+        while (it.next()) |p| {
+            if (p.*.parent == &pkg_tree_root) {
+                p.*.parent = pkg;
+            }
+        }
         pkg.table = pkg_tree_root.table;
         pkg_tree_root.table = .{};
+    } else {
+        // Remove any dangling pointers just in case.
+        var it = pkg_tree_root.table.valueIterator();
+        while (it.next()) |p| {
+            if (p.*.parent == &pkg_tree_root) {
+                p.*.parent = null;
+            }
+        }
     }
 
     const self_exe_path = try introspect.findZigExePath(arena);