[modular_test] no longer generate .packages within modular test suites

Since the null-safety test migration we've been generating both a .packages file
and a package_config.json file. This deletes the use of the old .packages file
and updates all references to use the new file.

Change-Id: Iecef64ac8ed8579338795ad5327765118d643236
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/240650
Reviewed-by: Joshua Litt <joshualitt@google.com>

.dart_tool/package_config.json

@@ -463,7 +463,7 @@
       "name": "modular_test",
       "rootUri": "../pkg/modular_test",
       "packageUri": "lib/",
-      "languageVersion": "2.2"
+      "languageVersion": "2.10"
     },
     {
       "name": "native_stack_traces",